Aaron J. Romanowsky
Condensed Bibliography |
Refereed Papers |
Unrefereed Papers |
Initial mass function variability (or not) among low-velocity dispersion, compact stellar systems
A. Villaume, J. Brodie, C. Conroy, A.J. Romanowsky, P. van Dokkum
The Astrophysical Journal Letters, submitted
[No abstract available.]
Deep Subaru Hyper Suprime-Cam observations of Milky Way satellites Columba I and Triangulum II
J.L. Carlin, D.J. Sand, R.R. Muñoz, K. Spekkens, B. Willman, D. Crnojević,
D.A. Forbes, J. Hargis, E. Kirby, A.H.G. Peter, A.J. Romanowsky, J. Strader
The Astrophysical Journal, submitted
[No abstract available.]
On the formation mechanisms of compact elliptical galaxies
A. Ferré-Mateu, D.A. Forbes, A.J. Romanowsky, J. Janz, C. Dixon
Monthly Notices of the Royal Astronomical Society, in press, arXiv:1709.07012
In order to investigate the formation mechanisms of the rare compact elliptical galaxies (cE) we have compiled a sample of 25 cEs with good SDSS spectra, covering a range of stellar masses, sizes and environments. They have been visually classified according to the interaction with their host, representing different evolutionary stages. We have included clearly disrupted galaxies, galaxies that despite not showing signs of interaction are located close to a massive neighbor (thus are good candidates for a stripping process), and cEs with no host nearby. For the latter, tidal stripping is less likely to have happened and instead they could simply represent the very low-mass, faint end of the ellipticals. We study a set of properties (structural parameters, stellar populations, star formation histories and mass ratios) that can be used to discriminate between an intrinsic or stripped origin. We find that one diagnostic tool alone is inconclusive for the majority of objects. However, if we combine all the tools a clear picture emerges. The most plausible origin, as well as the evolutionary stage and progenitor type, can be then determined. Our results favor the stripping mechanism for those galaxies in groups and clusters that have a plausible host nearby, but favors an intrinsic origin for those rare cEs without a plausible host and that are located in looser environments.
Extensive globular cluster systems associated with ultra diffuse galaxies in the Coma cluster
P. van Dokkum, R. Abraham, A.J. Romanowsky, J. Brodie, C. Conroy, S. Danieli, D. Lokhorst, A. Merritt, L. Mowla, J. Zhang
The Astrophysical Journal Letters, Volume 844, Number 1, 2017 July 20, Article L11 (7pp)
We present Hubble Space Telescope (HST) imaging of two ultra diffuse galaxies
(UDGs) with measured stellar velocity dispersions in the Coma cluster. The galaxies,
Dragonfly 44 and DFX1, have effective radii of 4.7 kpc and 3.5 kpc and velocity
dispersions of 47+8–6 km s– and
30+7–7 km s–, respectively. Both
galaxies are associated with a striking number of compact objects, tentatively
identified as globular clusters: Ngc = 74 ± 18 for Dragonfly 44
and Ngc = 62 ± 17 for DFX1. The number of globular clusters is
much higher than expected from the luminosities of the galaxies but is consistent
with expectations from the empirical relation between dynamical mass and globular
cluster count defined by other galaxies. Combining our data with previous HST
observations of Coma UDGs we find that UDGs have a factor of
6.9+1.0-2.4 more globular clusters than other galaxies of the
same luminosity, in contrast to a recent study of a similar sample by Amorisco et
al., but consistent with earlier results for individual galaxies. The Harris et al.
relation between globular cluster count and dark matter
halo mass implies a median halo mass of Mhalo ~ 1.5 ×
1011 M☉ for the sixteen Coma UDGs that have been
observed with HST so far, with the largest and brightest having
Mhalo ~ 5 × 1011 M☉.
Constraining the physical state of the hot gas halos in NGC 4649 and NGC 5846
A. Paggi, D.-W. Kim, C. Anderson, D. Burke, R. D'Abrusco, G. Fabbiano, A. Fruscione,
T. Gokas, J. Lauer, M. McCollough, D. Morgan, A. Mossman, E. O'Sullivan,
G. Trinchieri, S. Vrtilek, S. Pellegrini, A.J. Romanowsky, J. Brodie
The Astrophysical Journal, Volume 844, Number 1, 2017 July 20, Article 5 (30pp)
We present results of a joint Chandra/XMM-Newton analysis of the early-type galaxies
NGC 4649 and NGC 5846 aimed at investigating differences between mass profiles derived
from X-ray data and those from optical data, to probe the state of the hot interstellar
medium (ISM) in these galaxies. If the hot ISM is at a given radius in hydrostatic
equilibrium (HE), the X-ray data can be used to measure the total enclosed mass of the
galaxy. Differences from optically derived mass distributions therefore yield
information about departures from HE in the hot halos. The X-ray mass profiles in
different angular sectors of NGC 4649 are generally smooth with no significant
azimuthal asymmetries within 12 kpc. Extrapolation of these profiles beyond this scale
yields results consistent with the optical estimate. However, in the central region
(r < 3 kpc) the X-ray data underpredict the enclosed mass, when compared with
the optical mass profiles. Consistent with previous results, we estimate a nonthermal
pressure component accounting for 30% of the gas pressure, likely linked to nuclear
activity. In NGC 5846 the X-ray mass profiles show significant azimuthal asymmetries,
especially in the NE direction. Comparison with optical mass profiles in this direction
suggests significant departures from HE, consistent with bulk gas compression and
decompression due to sloshing on ~15 kpc scales; this effect disappears in the NW
direction, where the emission is smooth and extended. In this sector we find consistent
X-ray and optical mass profiles, suggesting that the hot halo is not responding to
strong nongravitational forces.
The SLUGGS Survey: trails of SLUGGS galaxies in a modified spin-ellipticity diagram
S. Bellstedt, A.W. Graham, D.A. Forbes, A.J. Romanowsky, J.P. Brodie, J. Strader
Monthly Notices of the Royal Astronomical Society, Volume 470, Issue 2, 2017 September, pp. 1321–1328
We present radial tracks for four early-type galaxies with embedded
intermediate-scale discs in a modified spin-ellipticity diagram. Here, each
galaxy's spin and ellipticity profiles are shown as a radial track, as opposed to
a single, flux-weighted aperture-dependent value as is common in the literature.
The use of a single ellipticity and spin parameter is inadequate to capture the
basic nature of these galaxies, which transition from fast to slow rotation as one
moves to larger radii where the disc ceases to dominate. After peaking, the four
galaxy's radial tracks feature a downturn in both ellipticity and spin with
increasing radius, differentiating them from elliptical galaxies, and from
lenticular galaxies whose discs dominate at large radii. These galaxies are
examples of so-called discy elliptical galaxies, which are a morphological hybrid
between elliptical (E) and lenticular (S0) galaxies and have been designated ES
galaxies. The use of spin-ellipticity tracks provides extra structural information
about individual galaxies over a single aperture measure. Such tracks provide a
key diagnostic for classifying early-type galaxies, particularly in the era of 2D
kinematic (and photometric) data beyond one effective radius.
The stellar initial mass function in early-type galaxies from absorption line spectroscopy. III. Radial gradients
P. van Dokkum, C. Conroy, A. Villaume, J. Brodie, A.J. Romanowsky
The Astrophysical Journal, Volume 841, Number 2, 2017 June 1, Article 68 (23pp)
There is good evidence that the centers of massive early-type galaxies have a bottom-heavy
stellar initial mass function (IMF) compared to that of the Milky Way. Here we study the
radial variation of the IMF within such galaxies, using a combination of high-quality Keck
spectroscopy and a new suite of stellar population synthesis models that cover a wide
range in metallicity. As in the previous studies in this series, the models are fitted
directly to the spectra and treat all elemental abundance ratios as free parameters. Using
newly obtained spectroscopy for six galaxies, including deep data extending to
~1 Re for the galaxies NGC 1407, NGC 1600, and NGC 2695, we find that
the IMF varies strongly with galactocentric radius. For all six galaxies the IMF is
bottom-heavy in the central regions, with average mass-to-light ratio "mismatch" parameter
α ≡ (M/L)/(M/L)MW ≈ 2.5
at R = 0. The IMF rapidly becomes more bottom-light with increasing radius,
flattening off near the Milky Way value (α ≈ 1.1) at R > 0.4
Re. A consequence is that the luminosity-weighted average IMF depends on
the measurement aperture: within R = Re we find
<α>L = 1.3–1.5, consistent with recent lensing and
dynamical results from SLACS and ATLAS3D. Our results are also consistent with
several earlier studies that were based on analyses of radial gradients of line indices.
The observed IMF gradients support galaxy formation models in which the central regions of
massive galaxies had a different formation history than their outer parts. Finally, we
make use of the high signal-to-noise central spectra of NGC 1407 and NGC 2695 to
demonstrate how we can disentangle IMF effects and abundance effects.
The HI content of isolated ultra-diffuse galaxies: A sign of multiple formation mechanisms?
E. Papastergis, E.A.K. Adams, A.J. Romanowsky
Astronomy & Astrophysics Letters, Volume 601, May 2017, Article L10 (4pp)
We report on the results of radio observations in the 21 cm emission line of atomic
hydrogen (HI) of four relatively isolated ultra-diffuse galaxies (UDGs): DGSAT I,
R-127-1, M-161-1, and SECCO-dI-2. Our Effelsberg observations resulted in
non-detections for the first three UDGs, and a clear detection for the last. DGSAT I,
R-127-1, and M-161-1 are quiescent galaxies with gas fractions that are much lower than
those of typical field galaxies of the same stellar mass. On the other hand,
SECCO-dI-2 is a star forming gas-rich dwarf, similar to two other field UDGs that have
literature HI data: SECCO-dI-1 and UGC 2162. This group of three gas-rich UDGs have
stellar and gaseous properties that are compatible with a recently proposed theoretical
mechanism for the formation of UDGs, based on feedback-driven outflows. In contrast,
the physical characteristics of R-127-1 and M-161-1 are puzzling, given their isolated
nature. We interpret this dichotomy in the gaseous properties of field UDGs as a sign
of the existence of multiple mechanisms for their formation, with the formation of the
quiescent gas-poor UDGs remaining a mystery.
The SLUGGS Survey: dark matter fractions at large radii and assembly epochs of early-type galaxies from globular cluster kinematics
A.B. Alabi, D.A. Forbes, A.J. Romanowsky, J.P. Brodie, J. Strader, J. Janz, C. Usher, L.R. Spitler, S. Bellstedt, A. Ferré-Mateu
Monthly Notices of the Royal Astronomical Society, Volume 468, Issue 4, 2017 July, pp. 3949–3964
We use globular cluster kinematics data, primarily from the SAGES Legacy Unifying Globulars
and GalaxieS (SLUGGS) survey, to measure the dark matter fraction (fDM)
and the average dark matter density (<ρDM>) within the inner 5
effective radii (Re) for 32 nearby early-type galaxies (ETGs) with
stellar mass log (M*/M☉) ranging from 10.1 to 11.8.
We compare our results with a simple galaxy model based on scaling relations as well as
with cosmological hydrodynamical simulations where the dark matter profile has been
modified through various physical processes. We find a high fDM
(≥ 0.6) within 5 Re in most of our sample, which we interpret as a
signature of a late mass assembly history that is largely devoid of gas-rich major mergers.
However, around log (M*/M☉) ~ 11, there is a wide range
of fDM which may be challenging to explain with any single cosmological
model. We find tentative evidence that lenticulars (S0s), unlike ellipticals, have mass
distributions that are similar to spiral galaxies, with decreasing fDM
within 5 Re as galaxy luminosity increases. However, we do not find any
difference between the <ρDM> of S0s and ellipticals in our sample,
despite the differences in their stellar populations. We have also used
<ρDM> to infer the epoch of halo assembly (z ~ 2–4).
By comparing the age of their central stars with the inferred epoch of halo formation, we
are able to gain more insight into their mass assembly histories. Our results suggest a
fundamental difference in the dominant late-phase mass assembly channel between lenticulars
and elliptical galaxies.
Detection of supermassive black holes in two Virgo ultracompact dwarf galaxies
C.P. Ahn, A.C. Seth, M. den Brok, J. Strader, H. Baumgardt, R. van den Bosch, I. Chilingarian, M. Frank, M. Hilker, R. McDermid, S. Mieske, A.J. Romanowsky, L. Spitler, J. Brodie, N. Neumayer, J.L. Walsh
The Astrophysical Journal, Volume 839, Number 2, 2017 April 20, Article 72 (15pp)
We present the detection of supermassive black holes (BHs) in two Virgo
ultracompact dwarf galaxies (UCDs), VUCD3 and M59cO. We use adaptive optics
assisted data from the Gemini/NIFS instrument to derive radial velocity dispersion
profiles for both objects. Mass models for the two UCDs are created using
multi-band Hubble Space Telescope imaging, including the modeling of mild
color gradients seen in both objects. We then find a best-fit stellar
mass-to-light ratio (M/L) and BH mass by combining the kinematic
data and the deprojected stellar mass profile using Jeans Anisotropic Models.
Assuming axisymmetric isotropic Jeans models, we detect BHs in both objects with
masses of 4.4+2.5–3.0 × 106
M☉ in VUCD3 and 5.8+2.5–2.8
× 106 M☉ in M59cO (3σ
uncertainties). The BH mass is degenerate with the anisotropy parameter,
βz; for the data to be consistent with no BH requires
βz = 0.4 and βz = 0.6 for VUCD3 and
M59cO, respectively. Comparing these values with nuclear star clusters shows that,
while it is possible that these UCDs are highly radially anisotropic, it seems
unlikely. These detections constitute the second and third UCDs known to host
supermassive BHs. They both have a high fraction of their total mass in their BH;
~13% for VUCD3 and ~18% for M59cO. They also have low best-fit stellar M/Ls,
supporting the proposed scenario that most massive UCDs host high-mass fraction
BHs. The properties of the BHs and UCDs are consistent with both objects being the
tidally stripped remnants of ~109 M☉ galaxies.
Ultra-diffuse and ultra-compact galaxies in the Frontier Fields cluster Abell 2744
S. Janssens, R. Abraham, J. Brodie, D. Forbes, A.J. Romanowsky, P. van Dokkum
The Astrophysical Journal Letters, Volume 839, Number 1, 2017 April 10, Article L17 (5pp)
We report the discovery of a large population of ultra-diffuse galaxies (UDGs) in the
massive galaxy cluster Abell 2744 (z = 0.308) as observed by the Hubble
Frontier Fields program. Since this cluster is ~5 times more massive than Coma, our
observations allow us to extend 0.7 dex beyond the high-mass end of the relationship
between UDG abundance and cluster mass reported by van der Burg et al. Using the same
selection criteria as van der Burg et al., A2744 hosts an estimated 1961 ± 577
UDGs, 10 times the number in Coma. As noted by Lee & Jang, A2744 contains numerous
unresolved compact objects, which those authors identified predominantly as globular
clusters. However, these objects have luminosities that are more consistent with
ultra-compact dwarf (UCD) galaxies. The abundances of both UCDs and UDGs scale with
cluster mass as a power law with a similar exponent, although UDGs and UCDs have very
different radial distributions within the cluster. The radial surface density
distribution of UCDs rises sharply toward the cluster center, while the surface
density distribution of the UDG population is essentially flat. Together, these
observations hint at a picture where some UCDs in A2744 may have once been associated
with infalling UDGs. As UDGs fall in and dissolve, they leave behind a residue of
The SLUGGS survey: using extended stellar kinematics to disentangle the
formation histories of low-mass S0 galaxies
S. Bellstedt, D.A. Forbes, C. Foster, A.J. Romanowsky, J.P. Brodie,
N. Pastorello, A. Alabi, A. Villaume
Monthly Notices of the Royal Astronomical Society, Volume 467, Issue 4, 2017 June, pp. 4540–4557
We utilize the DEIMOS instrument on the Keck telescope to measure the wide-field stellar
kinematics of early-type galaxies as part of the SAGES Legacy Unifying Globulars and GalaxieS
(SLUGGS) survey. In this paper, we focus on some of the lowest stellar mass lenticular galaxies
within this survey, namely NGC 2549, NGC 4474, NGC 4459 and NGC 7457, performing detailed
kinematic analyses out to large radial distances of ~2–3 effective radii. For NGC 2549,
we present the first analysis of data taken with the SuperSKiMS (Stellar Kinematics from
Multiple Slits) technique. To better probe kinematic variations in the outskirts of the SLUGGS
galaxies, we have defined a local measure of stellar spin. We use this parameter and identify
a clear separation in the radial behaviour of stellar spin between lenticular and elliptical
galaxies. We compare the kinematic properties of our galaxies with those from various simulated
galaxies to extract plausible formation scenarios. By doing this for multiple simulations, we
assess the consistency of the theoretical results. Comparisons to binary merger simulations
show that low-mass lenticular galaxies generally resemble the spiral progenitors more than the
merger remnants themselves, which is an indication that these galaxies are not formed through
merger events. We find, however, that recent mergers cannot be ruled out for some lenticular
The SLUGGS Survey: a catalog of over 4000 globular cluster radial
velocities in 27 nearby early-type galaxies
D.A. Forbes, A. Alabi, J.P. Brodie, A.J. Romanowsky, J. Strader, C. Foster,
C. Usher, L. Spitler, S. Bellstedt, N. Pastorello, A. Villaume, A. Wasserman, V. Pota
The Astronomical Journal, Volume 153, Number 3, 2017 March, Article 114 (10pp)
Here, we present positions and radial velocities for over 4000 globular clusters (GCs) in 27
nearby early-type galaxies from the SLUGGS survey. The SLUGGS survey is designed to be
representative of elliptical and lenticular galaxies in the stellar mass range 10 < log
M*/M☉ < 11.7. The data have been obtained over many years,
mostly using the very stable multi-object spectrograph DEIMOS on the Keck II 10 m telescope.
Radial velocities are measured using the calcium triplet lines, with a velocity accuracy of
±10–15 km s–1. We use phase space diagrams (i.e.,
velocity–position diagrams) to identify contaminants such as foreground stars and
background galaxies, and to show that the contribution of GCs from neighboring galaxies is
generally insignificant. Likely ultra-compact dwarfs are tabulated separately. We find that
the mean velocity of the GC system is close to that of the host galaxy systemic velocity,
indicating that the GC system is in overall dynamical equilibrium within the galaxy potential.
We also find that the GC system velocity dispersion scales with host galaxy stellar mass,
in a similar manner to the Faber–Jackson relation for the stellar velocity dispersion.
Publication of these GC radial velocity catalogs should enable further studies in many areas,
such as GC system substructure, kinematics, and host galaxy mass measurements.
The SLUGGS Survey: stellar masses and effective radii of early-type galaxies
from Spitzer Space Telescope 3.6 μm imaging
D.A. Forbes, L. Sinpetru, G. Savorgnan, A.J. Romanowsky, C. Usher, J. Brodie
Monthly Notices of the Royal Astronomical Society, Volume 464, Issue 4, 2017 February 01, pp. 4611–4623
Galaxy starlight at 3.6 μm is an excellent tracer of stellar mass. Here we use the latest 3.6 μm
imaging from the Spitzer Space Telescope to measure the total stellar mass and effective radii
in a homogeneous way for a sample of galaxies from the SAGES Legacy Unifying Globulars and GalaxieS
(SLUGGS) survey. These galaxies are representative of nearby early-type galaxies in the stellar mass
range of 10 < log M*/M☉ < 11.7 and our methodology can be applied
to other samples of early-type galaxies. We model each galaxy in 2D and estimate its total asymptotic
magnitude from a 1D curve-of-growth. Magnitudes are converted into stellar masses using a 3.6 μm
mass-to-light ratio from the latest stellar population models of Röck et al., assuming a Kroupa
initial mass function. We apply a ratio based on each galaxy's mean mass-weighted stellar age within
one effective radius (the mass-to-light ratio is insensitive to galaxy metallicity for the generally
old stellar ages and high metallicities found in massive early-type galaxies). Our 3.6 μm stellar
masses agree well with masses derived from 2.2 μm data. From the 1D surface brightness profile,
we fit a single Sérsic law, excluding the very central regions. We measure the effective
radius, Sérsic n parameter and effective surface brightness for each galaxy. We find
that galaxy sizes derived from shallow optical imaging and the 2MASS survey tend to underestimate
the true size of the largest, most massive galaxies in our sample. We adopt the 3.6 μm stellar
masses and effective radii for the SLUGGS survey galaxies.
Stellar populations across the black hole mass–velocity dispersion relation
I. Martín-Navarro, J.P. Brodie, R.C.E. van den Bosch, A.J. Romanowsky, D.A. Forbes
The Astrophysical Journal Letters, Volume 832, Number 1, 2016 November 20, Article L11 (5pp)
Coevolution between supermassive black holes (BH) and their host galaxies is universally adopted
in models for galaxy formation. In the absence of feedback from active galactic nuclei (AGNs),
simulated massive galaxies keep forming stars in the local universe. From an observational point
of view, however, such coevolution remains unclear. We present a stellar population analysis of
galaxies with direct BH mass measurements and the BH mass–σ relation as a
working framework. We find that over-massive BH galaxies, i.e., galaxies lying above the
best-fitting BH mass–σ line, tend to be older and more
α-element-enhanced than under-massive BH galaxies. The scatter in the BH
mass–[α/Fe] plane is significantly lower than that in the standard BH
mass–σ relation. We interpret this trend as an imprint of AGN feedback on
the star formation histories of massive galaxies.
Ultraluminous X-ray bursts in two ultracompact companions to nearby elliptical galaxies
J.A. Irwin, W.P. Maksym, G.R. Sivakoff, A.J. Romanowsky, D. Lin, T. Speegle, I. Prado, D. Mildebrath, J. Strader, J. Liu, J.M. Miller
Nature, Volume 538, Number 7625, 2016 October 20, pp. 356–358
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A flaring X-ray source was found near the galaxy NGC 4697
(ref. 1). Two brief flares were seen, separated by four years. During
each flare, the flux increased by a factor of 90 on a timescale of about
one minute. There is no associated optical source at the position
of the flares, but if the source was at the distance of NGC 4697,
then the luminosities of the flares were greater than 1039 erg per
second. Here we report the results of a search of archival X-ray
data for 70 nearby galaxies looking for similar flares. We found two
ultraluminous flaring sources in globular clusters or ultracompact
dwarf companions of parent elliptical galaxies. One source flared
once to a peak luminosity of 9×1040 erg per second; the other flared
five times to 1040 erg per second. The rise times of all of the flares
were less than one minute, and the flares then decayed over about an
hour. When not flaring, the sources appear to be normal accreting
neutron-star or black-hole X-ray binaries, but they are located in old
stellar populations, unlike the magnetars, anomalous X-ray pulsars
or soft γ repeaters that have repetitive flares of similar luminosities.
The SLUGGS Survey: revisiting the correlation between X-ray luminosity and
total mass of massive early-type galaxies
D.A. Forbes, A. Alabi, A.J. Romanowsky, D.-W. Kim, J.P. Brodie, G. Fabbiano
Monthly Notices Letters of the Royal Astronomical Society, Volume 464, Issue 1, 2017 January 01, pp. L26–L30
Here we utilize recent measures of galaxy total dynamical mass and X-ray gas luminosities
(LX,Gas) for a sample of 29 massive early-type galaxies from the SLUGGS survey to probe
LX,Gas–mass scaling relations. In particular, we investigate scalings with stellar mass, dynamical
mass within 5 effective radii (Re) and total virial mass. We also compare these relations
with predictions from Λ cold dark matter simulations. We find a strong linear relationship
between LX,Gas and galaxy dynamical mass within 5Re, which is consistent with the recent
cosmological simulations of Choi et al. that incorporate mechanical heating from AGN. We
conclude that the gas surrounding massive early-type galaxies was shock-heated as it fell into
collapsing dark matter haloes so that LX,Gas is primarily driven by the depth of a galaxy's
potential well. Heating by an AGN plays an important secondary role in determining LX,Gas.
Metallicity and age of the stellar stream around the disk galaxy NGC 5907
S. Laine, C.J. Grillmair, P. Capak, R.G. Arendt, A.J. Romanowsky, D. Martínez-Delgado,
M.L.N. Ashby, J.E. Davies, S.R. Majewski, J.P. Brodie, R.J. GaBany, J.A. Arnold
The Astronomical Journal, Volume 152, Number 3, 2016 September, Article 72 (11pp)
Stellar streams have become central to studies of the interaction histories of nearby galaxies.
To characterize the most prominent parts of the stellar stream around the well-known nearby
(d = 17 Mpc) edge-on disk galaxy NGC 5907, we have obtained and analyzed new, deep
gri Subaru/Suprime-Cam and 3.6 μm Spitzer/Infrared Array Camera
observations. Combining the near-infrared 3.6 μm data with visible-light images allows
us to use a long wavelength baseline to estimate the metallicity and age of the stellar
population along an ~60 kpc long segment of the stream. We have fitted the stellar spectral
energy distribution with a single-burst stellar population synthesis model and we use it to
distinguish between the proposed satellite accretion and minor/major merger formation models of
the stellar stream around this galaxy. We conclude that a massive minor merger (stellar mass
ratio of at least 1:8) can best account for the metallicity of –0.3 inferred along the
brightest parts of the stream.
A discrete chemo-dynamical model of the giant elliptical galaxy NGC 5846:
dark matter fraction, internal rotation and velocity anisotropy out
to six effective radii
L. Zhu, A.J. Romanowsky, G. van de Ven, R.J. Long, L.L Watkins,
V. Pota, N.R. Napolitano, D.A. Forbes, J. Brodie, C. Foster
Monthly Notices of the Royal Astronomical Society, Volume 462, Issue 4, 2016 November 11, pp. 4001–4017
We construct a suite of discrete chemo-dynamical models of the giant elliptical galaxy NGC 5846. These models are a powerful
tool to constrain both the mass distribution and internal dynamics of multiple tracer populations. We use Jeans models to
simultaneously fit stellar kinematics within the effective radius Re, planetary nebula (PN) radial velocities out to
3 Re, and globular cluster (GC) radial velocities and colours out to 6 Re. The best-fitting
model is a cored dark matter halo which contributes ~10 per cent of the total mass within 1 Re, and
67 per cent ± 10 per cent within 6 Re, although a cusped dark matter halo is also acceptable. The red GCs
exhibit mild rotation with vmax/σ0 ~ 0.3 in the region
R > Re, aligned with but counter-rotating to the stars in the inner parts, while the blue GCs and PNe
kinematics are consistent with no rotation. The red GCs are tangentially anisotropic, the blue GCs are mildly radially
anisotropic, and the PNe vary from radially to tangentially anisotropic from the inner to the outer region. This is confirmed
by general made-to-measure models. The tangential anisotropy of the red GCs in the inner regions could stem from the
preferential destruction of red GCs on more radial orbits, while their outer tangential anisotropy – similar to the PNe
in this region – has no good explanation. The mild radial anisotropy of the blue GCs is consistent with an accretion
A high stellar velocity dispersion and ~100 globular clusters in the
ultra-diffuse galaxy Dragonfly 44
P. van Dokkum, R. Abraham, J. Brodie, C. Conroy, S. Danieli, A. Merritt, L. Mowla,
A. Romanowsky, J. Zhang
The Astrophysical Journal Letters, Volume 828, Number 1, 2016 September 1, Article L6 (6pp)
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Recently a population of large, very low surface brightness, spheroidal galaxies was identified in the Coma cluster. The
apparent survival of these ultra-diffuse galaxies (UDGs) in a rich cluster suggests that they have very high masses. Here, we
present the stellar kinematics of Dragonfly 44, one of the largest Coma UDGs, using a 33.5 hr integration with DEIMOS on the
Keck II telescope. We find a velocity dispersion of σ = 47+8–-6
km s–1, which implies a dynamical mass of Mdyn(< r1/2) =
0.7+0.3–0.2 × 1010 M☉ within its deprojected half-light radius
of r1/2 = 4.6 ± 0.2 kpc. The mass-to-light ratio is M/LI(< r1/2)
= 48+21–14 M☉/L☉, and the dark matter fraction is 98% within
r1/2. The high mass of Dragonfly 44 is accompanied by a large globular cluster population. From deep Gemini
imaging taken in 0.4" seeing we infer that Dragonfly 44 has 94+25–20 globular clusters, similar to
the counts for other galaxies in this mass range. Our results add to other recent evidence that many UDGs are "failed"
galaxies, with the sizes, dark matter content, and globular cluster systems of much more luminous objects. We estimate the
total dark halo mass of Dragonfly 44 by comparing the amount of dark matter within r = 4.6 kpc to enclosed mass
profiles of NFW halos. The enclosed mass suggests a total mass of ~1012 M☉, similar to the mass of
the Milky Way. The existence of nearly dark objects with this mass is unexpected, as galaxy formation is thought to be
maximally efficient in this regime.
First results from the MADCASH Survey: a faint dwarf galaxy companion
to the low-mass spiral galaxy NGC 2403 at 3.2 Mpc
J.L. Carlin, D.J. Sand, P. Price, B. Willman, A. Karunakaran,
K. Spekkens, E.F. Bell, J.P. Brodie, Crnojević, D.A. Forbes, J. Hargis,
E.K. Kirby, R. Lupton, A.H.G. Peter, A.J. Romanowsky, J. Strader
The Astrophysical Journal Letters, Volume 828, Number 1, 2016 September 1, Article L5 (6pp)
We report the discovery of the faintest known dwarf galaxy satellite of a Large Magellanic Cloud (LMC) stellar-mass host
beyond the Local Group (LG), based on deep imaging with Subaru/Hyper Suprime-Cam. Magellanic Analog Dwarf Companions And
Stellar Halos (MADCASH) J074238+652501-dw lies ~35 kpc in projection from NGC 2403, a dwarf spiral galaxy at D
≈ 3.2 Mpc. This new dwarf has Mg = –7.4 ± 0.4 and a half-light radius of 168 ± 70 pc, at
the calculated distance of 3.39 ± 0.41 Mpc. The color–magnitude diagram reveals no evidence of young stellar
populations, suggesting that MADCASH J074238+652501-dw is an old, metal-poor dwarf similar to low-luminosity dwarfs in the LG.
The lack of either detected HI gas (MHI/LV < 0.69 M☉/L☉,
based on Green Bank Telescope observations) or GALEX NUV/FUV flux enhancement is consistent with a lack of young stars.
This is the first result from the MADCASH survey, which is conducting a census of the stellar substructure and faint satellites
in the halos of Local Volume LMC analogs via resolved stellar populations. Models predict a total of ~4–10 satellites at
least as massive as MADCASH J074238+652501-dw around a host with the mass of NGC 2403, with 2–3 within our field of view,
slightly more than the one such satellite observed in our footprint.
The SLUGGS survey: a new mask design to reconstruct the stellar populations and
kinematics of both inner and outer galaxy regions
N. Pastorello, D.A. Forbes, A. Poci, A.J. Romanowsky, R. McDermid,
A.B. Alabi, J.P. Brodie, M. Cappellari, V. Pota, C. Foster
Publications of the Astronomical Society of Australia, Volume 33, 2016, e035 (16 pp)
Integral field unit spectrographs allow the 2D exploration of the kinematics and stellar populations of galaxies, although
they are generally restricted to small fields-of-view. Using the large field-of-view of the DEIMOS multislit spectrograph on
Keck and our Stellar Kinematics using Multiple Slits technique, we are able to extract sky-subtracted stellar light spectra to
large galactocentric radii. Here, we present a new DEIMOS mask design named SuperSKiMS that explores large spatial
scales without sacrificing high spatial sampling. We simulate a set of observations with such a mask design on the nearby
galaxy NGC 1023, measuring stellar kinematics and metallicities out to where the galaxy surface brightness is orders of
magnitude fainter than the sky. With this technique we also reproduce the results from literature integral field spectroscopy
in the innermost galaxy regions. In particular, we use the simulated NGC 1023 kinematics to model its total mass distribution
to large radii, obtaining comparable results with those from published integral field unit observation. Finally, from new
spectra of NGC 1023, we obtain stellar 2D kinematics and metallicity distributions that show good agreement with integral
field spectroscopy results in the overlapping regions. In particular, we do not find a significant offset between our Stellar
Kinematics using Multiple Slits and the ATLAS3D stellar velocity dispersion at the same spatial locations.
The mass discrepancy acceleration relation in early-type galaxies: extended mass profiles and the phantom menace to MOND
J. Janz, M. Cappellari, A.J. Romanowsky, L. Ciotti, A. Alabi, D.A. Forbes
Monthly Notices of the Royal Astronomical Society, Volume 461, Issue 3, 2016 September 21, pp. 2367–2373
The dark matter (DM) haloes around spiral galaxies appear to conspire with
their baryonic content: empirically, significant amounts of DM are inferred
only below a universal characteristic acceleration scale. Moreover, the
discrepancy between the baryonic and dynamical mass, which is usually
interpreted as the presence of DM, follows a very tight mass discrepancy
acceleration (MDA) relation. Its universality, and its tightness in spiral
galaxies, poses a challenge for the DM interpretation and was used to argue
in favour of MOdified Newtonian Dynamics (MOND). Here, we test whether or
not this applies to early-type galaxies. We use the dynamical models of
fast-rotator early-type galaxies by Cappellari et al. based on
ATLAS3D and SAGES Legacy Unifying Globulars and GalaxieS
(SLUGGS) data, which was the first homogenous study of this kind, reaching
~4 Re, where DM begins to dominate the total mass budget.
We find the early-type galaxies to follow an MDA relation similar to spiral
galaxies, but systematically offset. Also, while the slopes of the mass
density profiles inferred from galaxy dynamics show consistency with those
expected from their stellar content assuming MOND, some profiles of
individual galaxies show discrepancies.
The SLUGGS Survey: the mass distribution in early-type galaxies within five effective radii and beyond
A. Alabi, D.A. Forbes, A.J. Romanowsky, J.P. Brodie, J. Strader, J. Janz,
V. Pota, N. Pastorello, C. Usher, L.R. Spitler, C. Foster, Z.G. Jennings,
A. Villaume, S. Kartha
Monthly Notices of the Royal Astronomical Society, Volume 460, Issue 4, 2016 August 21, pp. 3838–3860
We study mass distributions within and beyond 5 effective radii (Re) in 23 early-type
galaxies from the SAGES Legacy Unifying Globulars and Galaxies Survey, using their globular cluster (GC)
kinematic data. The data are obtained with Keck/DEep Imaging Multi-Object Spectrograph, and consist of
line-of-sight velocities for ~3500 GCs, measured with a high precision of ~15 km s–1 per
GC and extending out to ~13 Re. We obtain the mass distribution in each galaxy using the
tracer mass estimator of Watkins et al. and account for kinematic substructures, rotation of the GC systems
and galaxy flattening in our mass estimates. The observed scatter between our mass estimates and results
from the literature is less than 0.2 dex. The dark matter fraction within 5 Re
(fDM) increases from ~0.6 to ~0.8 for low- and high-mass galaxies, respectively, with
some intermediate-mass galaxies (M★ ~ 1011 M☉)
having low fDM ~ 0.3, which appears at odds with predictions from simple galaxy models.
We show that these results are independent of the adopted orbital anisotropy, stellar mass-to-light
(M/L) ratio, and the assumed slope of the gravitational potential. However, the low
fDM in the ~1011 M☉ galaxies agrees with the cosmological
simulations of Wu et al. where the pristine dark matter distribution has been modified by baryons during
the galaxy assembly process. We find hints that these M★ ~ 1011
M☉ galaxies with low fDM have very diffuse dark matter haloes, implying
that they assembled late. Beyond 5 Re, the M/L gradients are steeper in the
more massive galaxies and shallower in both low and intermediate mass galaxies.
Star clusters in M31: VII. Global kinematics and metallicity subpopulations of the globular clusters
N. Caldwell, A.J. Romanowsky
The Astrophysical Journal, Volume 824, Number 1, 2016 June 10, Article 42 (8pp)
We carry out a joint spatial–kinematical–metallicity analysis of globular clusters (GCs)
around the Andromeda Galaxy (M31), using a homogeneous, high-quality spectroscopic data set. In particular, we remove the contaminating young clusters that have plagued many previous analyses. We find that the
clusters can be divided into three major metallicity groups based on their radial distributions: (1) an
inner metal-rich group ([Fe/H] > –0.4); (2) a group with intermediate metallicity (with median [Fe/H]
= –1); and (3) a metal-poor group, with [Fe/H] < –1.5. The metal-rich group has kinematics and
spatial properties like those of the disk of M31, while the two more metal-poor groups show mild prograde
rotation overall, with larger dispersions—in contrast to previous claims of stronger rotation. The
metal-poor GCs are the least concentrated group; such clusters occur five times less frequently in the
central bulge than do clusters of higher metallicity. Despite some well-known differences between the M31
and Milky Way GC systems, our revised analysis points to remarkable similarities in their chemodynamical
properties, which could help elucidate the different formation stages of galaxies and their GCs. In
particular, the M31 results motivate further exploration of a metal-rich GC formation mode in situ, within
high-redshift, clumpy galactic disks.
New spectroscopic technique based on coaddition of surface brightness fluctuations:
NGC4449 and its stellar tidal stream
E. Toloba, P. Guhathakurta, A.J. Romanowsky, J.P. Brodie, D. Martínez-Delgado, J.A. Arnold, N. Ramachandran, K. Theakanath
The Astrophysical Journal, Volume 824, Number 1, 2016 June 10, Article 35 (11pp)
We present a new spectroscopic technique based in part on targeting the upward fluctuations of the surface
brightness for studying the internal stellar kinematics and metallicities of galaxies of low surface
brightness effects both to galaxies and streams beyond the Local Group. The distance to these systems
makes them unsuitable for targeting individual red giant branch (RGB) stars (tip of RGB at I >~ 24
mag) and their surface brightness is too low (μr >~ 25 mag arcsec–2)
for integrated light spectroscopic measurements. This technique overcomes these two problems by targeting
individual objects that are brighter than the tip of the RGB. We apply this technique to the star-forming
dwarf galaxy NGC 4449 and its stellar stream. We use Keck/DEIMOS data to measure the line-of-sight radial
velocity out to ~7 kpc in the east side of the galaxy and ~8 kpc along the stream. We find that the two
systems are likely gravitationally bound to each other and have heliocentric radial velocities of 227.3
± 10.7 km s–1 and 225.8 ± 16.0 km s–1, respectively. Neither the
stream nor the near half of the galaxy shows a significant velocity gradient. We estimate the stellar
metallicity of the stream based on the equivalent width of its calcium triplet lines and find [Fe/H] =
–1.37 ± 0.41, which is consistent with the metallicity–luminosity relation for dwarf
galaxies in the Local Group. Whether the stream's progenitor was moderately or severely stripped cannot be
constrained with this uncertainty in metallicity. We demonstrate that this new technique can be used to
measure the kinematics and (possibly) the metallicity of the numerous faint satellites and stellar streams
in the halos of nearby (~4 Mpc) galaxies.
Discovery of the candidate off-nuclear ultrasoft hyper-luminous X-ray source 3XMM J141711.1+522541
D. Lin, E.R. Carrasco, N.A. Webb, J.A. Irwin, R. Dupke, A.J. Romanowsky, E. Ramirez-Ruiz, J. Strader, J. Homan, D. Barret, O. Godet
The Astrophysical Journal, Volume 821, Number 1, 2016 April 10, Article 25 (12pp)
We report the discovery of an off-nuclear ultrasoft hyper-luminous X-ray source candidate 3XMM
J141711.1+522541 in the inactive S0 galaxy SDSS J141711.07+522540.8 (z = 0.41827,
dL = 2.3 Gpc) in the Extended Groth Strip. It is located at a projected offset of
~1.0" (5.2 kpc) from the nucleus of the galaxy and was serendipitously detected in five XMM-Newton
observations in 2000 July. Two observations have enough counts and can be fitted with a standard
thermal disk with an apparent inner disk temperature kTMCD ~ 0.13 keV and a
0.28–14.2 keV unabsorbed luminosity LX ~ 4×1043
erg s–1 in the source rest frame. The source was still detected in three
Chandra observations in 2002 August, with similarly ultrasoft but fainter spectra
(kTMCD ~ 0.17 keV, LX ~ 0.5×1043 erg
s–1). It was not detected in later observations, including two by Chandra
in 2005 October, one by XMM-Newton in 2014 January, and two by Chandra in 2014
September–October, implying a long-term flux variation factor of >14. Therefore the source
could be a transient with an outburst in 2000–2002. It has a faint optical counterpart
candidate, with apparent magnitudes of mF606W = 26.3 AB mag and
mF814W = 25.5 AB mag in 2004 December (implying an absolute V-band
magnitude of ~ –15.9 AB mag). We discuss various explanations for the source and find that it
is best explained as a massive black hole (BH) embedded in the nucleus of a possibly stripped
satellite galaxy, with the X-ray outburst due to tidal disruption of a surrounding star by the BH.
The BH mass is ~105 M☉, assuming the peak X-ray luminosity at
around the Eddington limit.
Discovery of an ultra-diffuse galaxy in the Pisces-Perseus supercluster
D. Martínez-Delgado, R. Läsker, M. Sharina, E. Toloba, J. Fliri, R. Beaton, D. Valls-Gabaud, I.D. Karachentsev, T.S. Chonis, E.K. Grebel,
D.A. Forbes, A.J. Romanowsky, J. Gallego-Laborda, K. Teuwen,
M.A. Gómez-Flechoso, J. Wang, P. Guhathakurta, S. Kaisin, N. Ho
The Astronomical Journal, Volume 151, Number 4, 2016 April, Article 96 (13pp)
We report the discovery of DGSAT I, an ultra-diffuse, quenched galaxy located
10.4° in projection from the Andromeda
galaxy (M31). This low-surface brightness galaxy (μV = 24.8 mag
arcsec–2), found with a small amateur telescope, appears unresolved
in sub-arcsecond archival Subaru/Suprime-Cam images, and hence has been missed by
optical surveys relying on resolved star counts, in spite of its relatively large
effective radius (Re(V) = 12") and proximity (15') to the
well-known dwarf spheroidal galaxy And II. Its red color (V–I = 1.0),
shallow Sérsic index (nV = 0.68), and the absence of detectable
Hα emission are typical properties of dwarf spheroidal galaxies and
suggest that it is mainly composed of old stars. Initially interpreted as an
interesting case of an isolated dwarf spheroidal galaxy in the local universe, our
radial velocity measurement obtained with the BTA 6 m telescope (Vh
= 5450 ± 40 km s–1) shows that this system is an M31-background
galaxy associated with the filament of the Pisces-Perseus supercluster. At the
distance of this cluster (~78 Mpc), DGSAT I would have an Re ~ 4.7
kpc and MV –16.3. Its properties resemble those of the
ultra-diffuse galaxies (UDGs) recently discovered in the Coma cluster. DGSAT I is the
first case of these rare UDGs found in this galaxy cluster. Unlike the UDGs associated
with the Coma and Virgo clusters, DGSAT I is found in a much lower density
environment, which provides a fresh constraint on the formation mechanisms for this
intriguing class of galaxy.
The SLUGGS survey: exploring the globular cluster systems of the Leo II group and their global relationships
S.S. Kartha, D.A. Forbes, A.B. Alabi, J.P. Brodie, A.J. Romanowsky,
J. Strader, L.R. Spitler, Z.G. Jennings, J.C. Roediger
Monthly Notices of the Royal Astronomical Society, Volume 458, Issue 1, 2016 May 01, pp. 105–126
We present an investigation of the globular cluster (GC) systems of NGC 3607 and
NGC 3608 as part of the ongoing SLUGGS (SAGES Legacy Unifying Globulars and
GalaxieS) survey. We use wide-field imaging data from the Subaru telescope in the
g, r and i filters to analyse the radial density, colour and
azimuthal distributions of both GC systems. With the complementary kinematic data
obtained from the Keck II telescope, we measure the radial velocities of a total
of 81 GCs. Our results show that the GC systems of NGC 3607 and NGC 3608 have a
detectable spatial extent of ~15 and 13 galaxy effective radii, respectively. Both
GC systems show a clear bimodal colour distribution. We detect a significant radial
colour gradient for the GC subpopulations in both galaxies. NGC 3607 exhibits an
overabundance of red GCs on the galaxy minor axis and NGC 3608 shows a misalignment
in the GC subpopulation position angles with respect to the galaxy stellar component.
With the aid of literature data, we discuss several relationships between the
properties of GC systems and their host galaxies. A one-to-one relation between the
ellipticities of red GCs and the galaxy stellar light emphasizes the evolutionary
similarities between them. In our sample of four slowly rotating galaxies with
kinematically decoupled cores, we observe a higher ellipticity for the blue GC
subpopulation than their red counterparts. Also, we notice the flattening of
negative colour gradients for the blue GC subpopulations with increasing galaxy
stellar mass. Finally, we discuss the formation scenarios associated with the blue
An overmassive dark halo around an ultra-diffuse galaxy in the Virgo cluster
M.A. Beasley, A.J. Romanowsky, V. Pota, I. Martin-Navarro, D. Martinez-Delgado, F. Neyer, A.L. Deich
The Astrophysical Journal Letters, Volume 819, Number 2, 2016 March 10, Article L20 (7pp)
AAS Nova |
New Scientist |
Ultra-diffuse galaxies (UDGs) have the sizes of giants but the luminosities of dwarfs. A key to
understanding their origins comes from their total masses, but their low surface brightnesses
(μ(V) ≥ 25.0) generally prohibit dynamical studies. Here, we report the
first such measurements for a UDG (VCC 1287 in the Virgo cluster), based on its globular cluster
system dynamics and size. From seven GCs we measure a mean systemic velocity
vsys = 1071+14–15 km s–1, thereby confirming a Virgo
cluster association. We measure a velocity dispersion of 33+16–10
km s–1 within 8.1 kpc, corresponding to an enclosed mass of (4.5 ± 2.8) ×
109 M☉ and a g-band mass-to-light ratio of
(M/L)g = 106+126–54 within an effective radius. From the cumulative mass curve, along with the GC numbers, we estimate a virial mass of ~8
×1010 M☉, yielding a dark-to-stellar mass fraction of ~3000. We show
that this UDG is an outlier in Mstar–Mhalo relations,
suggesting extreme stochasticity in relatively massive star-forming halos in clusters. Finally, we
discuss how counting GCs offers an efficient route to determining virial masses for UDGs.
The SLUGGS survey:
globular clusters and the dark matter content of early-type galaxies
D.A. Forbes, A. Alabi, A.J. Romanowsky, J.P. Brodie, J. Strader, C. Usher, V. Pota
Monthly Notices Letters of the Royal Astronomical Society, Volume 458, Issue 1, 2016 May 01, pp. L44–L48
A strong correlation exists between the total mass of a globular cluster (GC) system and the
virial halo mass of the host galaxy. However, the total halo mass in this correlation is a
statistical measure conducted on spatial scales that are some 10 times that of a typical GC
system. Here we investigate the connection between GC systems and galaxy's dark matter on
comparable spatial scales, using dynamical masses measured on a galaxy-by-galaxy basis. Our
sample consists of 17 well-studied massive (~1011 M☉) early-type
galaxies from the SLUGGS survey. We find the strongest correlation to be that of the blue
(metal-poor) GC subpopulation and the dark matter content. This correlation implies that the
dark matter mass of a galaxy can be estimated to within a factor of 2 from careful imaging
of its GC system. The ratio of the GC system mass to that of the enclosed dark matter is
nearly constant. We also find a strong correlation between the fraction of blue GCs and the
fraction of enclosed dark matter, so that a typical galaxy with a blue GC fraction of 60 per
cent has a dark matter fraction of 86 per cent over similar spatial scales. Both halo growth
and removal (via tidal stripping) may play some role in shaping this trend. In the context
of the two-phase model for galaxy formation, we find galaxies with the highest fractions of
accreted stars to have higher dark matter fractions for a given fraction of blue GCs.
The SLUGGS Survey: the assembly histories of individual early-type galaxies
D.A. Forbes, A.J. Romanowsky, N. Pastorello, C. Foster,
J.P. Brodie, J. Strader, C. Usher, V. Pota
Monthly Notices of the Royal Astronomical Society, Volume 457, Issue 2, 2016 April 01, pp. 1242–1256
Early-type (E and S0) galaxies may have assembled via a variety of different
evolutionary pathways. Here, we investigate these pathways by comparing the
stellar kinematic properties of 24 early-type galaxies from the SAGES Legacy
Unifying Globulars and GalaxieS (SLUGGS) survey with the hydrodynamical
simulations of Naab et al. In particular, we use the kinematics of starlight
up to 4 effective radii (Re) as diagnostics of galaxy inner
and outer regions, and assign each galaxy to one of six Naab et al. assembly
classes. The majority of our galaxies (14/24) have kinematic characteristics
that indicate an assembly history dominated by gradual gas dissipation and
accretion of many gas-rich minor mergers. Three galaxies, all S0s, indicate
that they have experienced gas-rich major mergers in their more recent past.
One additional elliptical galaxy is tentatively associated with a gas-rich
merger which results in a remnant galaxy with low angular momentum. Pathways
dominated by gas-poor (major or minor) mergers dominate the mass growth of six
galaxies. Most SLUGGS galaxies appear to have grown in mass (and size) via the
accretion of stars and gas from minor mergers, with late major mergers playing
a much smaller role. We find that the fraction of accreted stars correlates
with the stellar mean age and metallicity gradient, but not with the slope of
the total mass density profile. We briefly mention future observational and
modelling approaches that will enhance our ability to accurately reconstruct
the assembly histories of individual present-day galaxies.
Satellite accretion in action: a tidally disrupting dwarf
spheroidal around the nearby spiral galaxy NGC 253
A.J. Romanowsky, D. Martínez-Delgado, N.F. Martin,
G. Morales, Z.G. Jennings, R.J. GaBany, J.P. Brodie, E.K. Grebel,
J. Schedler, M. Sidonio
Monthly Notices Letters of the Royal Astronomical Society, Volume 457, Issue 1, 2016 March 21, pp. L103–L107
New Scientist |
Sky & Telescope |
Canberra Times |
ABC Australia |
Sunrise TV |
We report the discovery of NGC 253-dw2, a dwarf spheroidal (dSph) galaxy candidate undergoing
tidal disruption around a nearby spiral galaxy, NGC 253 in the Sculptor group: the first such
event identified beyond the Local Group. The dwarf was found using small-aperture amateur
telescopes, and followed up with Suprime-Cam on the 8 m Subaru Telescope in order to resolve
its brightest stars. Using g- and RC-band photometry, we detect a red
giant branch consistent with an old, metal-poor stellar population at a distance of ~3.5 Mpc.
From the distribution of likely member stars, we infer a highly elongated shape with a
semimajor axis half-light radius of (2 ± 0.4) kpc. Star counts also yield a luminosity
estimate of ~2×106 L☉,V (MV
~ –10.7). The morphological properties of NGC 253-dw2 mark it as distinct from normal
dSphs and imply ongoing disruption at a projected distance of ~50 kpc from the main galaxy.
Our observations support the hierarchical paradigm wherein massive galaxies continuously
accrete less massive ones, and provide a new case study for dSph infall and dissolution
dynamics. We also note the continued efficacy of small telescopes for making big discoveries.
The SLUGGS Survey: stellar kinematics, kinemetry and trends at large radii in 25 early-type galaxies
C. Foster, N. Pastorello, J. Roediger, J.P. Brodie, D.A. Forbes, S.S. Kartha, V. Pota, A.J. Romanowsky,
L.R. Spitler, J. Strader, C. Usher, J.A. Arnold
Monthly Notices of the Royal Astronomical Society, Volume 457, Issue 1, 2016 March 21, pp. 147–171
Due to longer dynamical time-scales, the outskirts of early-type galaxies retain the footprint
of their formation and assembly. Under the popular two-phase galaxy formation scenario, an
initial in situ phase of star formation is followed by minor merging and accretion of ex
situ stars leading to the expectation of observable transitions in the kinematics and stellar
populations on large scales. However, observing the faint galactic outskirts is challenging,
often leaving the transition unexplored. The large-scale, spatially resolved stellar kinematic
data from the SAGES Legacy Unifying Galaxies and GlobularS (SLUGGS) survey are ideal for
detecting kinematic transitions. We present kinematic maps out to 2.6 effective radii on
average, kinemetry profiles, measurement of kinematic twists and misalignments, and the average
outer intrinsic shape of 25 SLUGGS galaxies. We find good overall agreement in the kinematic
maps and kinemetry radial profiles with literature. We are able to confirm significant radial
modulations in rotational versus pressure support of galaxies with radius so that the central
and outer rotational properties may be quite different. We also test the suggestion that galaxies
may be more triaxial in their outskirts and find that while fast rotating galaxies were already
shown to be axisymmetric in their inner regions, we are unable to rule out triaxiality in their
outskirts. We compare our derived outer kinematic information to model predictions from a
two-phase galaxy formation scenario. We find that the theoretical range of local outer angular
momentum agrees well with our observations, but that radial modulations are much smaller than
The SLUGGS survey: chromodynamical modelling of the lenticular galaxy NGC 1023
A. Cortesi, A.L. Chies-Santos, V. Pota, C. Foster, L. Coccato,
C. Mendes de Oliveira, D.A. Forbes, M.M. Merrifield, S.P. Bamford,
A.J. Romanowsky, J.P. Brodie, S.S. Kartha, A.B. Alabi,
R.N. Proctor, A. Almeida
Monthly Notices of the Royal Astronomical Society, Volume 456, Issue 3, 2016 March 01, pp. 2611–2621
Globular clusters (GCs) can be considered discrete, long-lived, dynamical
tracers that retain crucial information about the assembly history of their
parent galaxy. In this paper, we present a new catalogue of GC velocities and
colours for the lenticular galaxy NGC 1023, we study their kinematics and
spatial distribution, in comparison with the underlying stellar kinematics
and surface brightness profile, and we test a new method for studying GC
properties. Specifically, we decompose the galaxy light into its spheroid
(assumed to represent the bulge+halo components) and disc components and use
it to assign to each GC a probability of belonging to one of the two
components. Then we model the galaxy kinematics, assuming a disc and
spheroidal component, using planetary nebulae and integrated stellar light.
We use this kinematic model and the probability previously obtained from the
photometry to recalculate for each GC its likelihood of being associated with
the disc, the spheroid, or neither. We find that the reddest GCs are likely
to be associated with the disc, as found for faint fuzzies in this same
galaxy, suggesting that the disc of this S0 galaxy originated at
z ≅ 2. The majority of blue GCs are found likely to be associated
with the spheroidal (hot) component. The method also allows us to identify
objects that are unlikely to be in equilibrium with the system. In NGC 1023
some of the rejected GCs form a substructure in phase space that is connected
with NGC 1023 companion galaxy.
The AIMSS Project – III. The stellar populations of compact stellar systems
J. Janz, M.A. Norris, D.A. Forbes, A. Huxor, A.J. Romanowsky, M.J. Frank,
C.G. Escudero, F.R. Faifer, J.C. Forte, S.J. Kannappan, C. Maraston,
J.P. Brodie, J. Strader, B.R. Thompson
Monthly Notices of the Royal Astronomical Society, Volume 456, Issue 1, 2016 February 11, pp. 617–632
In recent years, a growing zoo of compact stellar systems (CSSs) have been
found whose physical properties (mass, size, velocity dispersion) place them
between classical globular clusters (GCs) and true galaxies, leading to
debates about their nature. Here we present results using a so far
underutilized discriminant, their stellar population properties. Based on new
spectroscopy from 8–10m telescopes, we derive ages, metallicities, and
[α/Fe] of 29 CSSs. These range from GCs with sizes of merely a few
parsec to compact ellipticals (cEs) larger than M32. Together with a
literature compilation, this provides a panoramic view of the stellar
population characteristics of early-type systems. We find that the CSSs are
predominantly more metal rich than typical galaxies at the same stellar mass.
At high mass, the cEs depart from the mass–metallicity relation of
massive early-type galaxies, which forms a continuous sequence with dwarf
galaxies. At lower mass, the metallicity distribution of ultracompact dwarfs
(UCDs) changes at a few times 107 M☉, which roughly
coincides with the mass where luminosity function arguments previously
suggested the GC population ends. The highest metallicities in CSSs are
paralleled only by those of dwarf galaxy nuclei and the central parts of
massive early types. These findings can be interpreted as CSSs previously
being more massive and undergoing tidal interactions to obtain their
current mass and compact size. Such an interpretation is supported by CSSs
with direct evidence for tidal stripping, and by an examination of the CSS
internal escape velocities.
NGC 3628-UCD1: a possible ω Cen analog embedded in a stellar stream
Z.G. Jennings, A.J. Romanowsky, J.P. Brodie, J. Janz, M.A. Norris, D.A. Forbes, D. Martinez-Delgado, M. Fagioli, S.J. Penny
The Astrophysical Journal Letters, Volume 812, Number 1, 2015 October 10, Article L10 (6pp)
Using Subaru/Suprime-Cam wide-field imaging and both Keck/ESI and LBT/MODS spectroscopy, we
identify and characterize a compact star cluster, which we term NGC 3628-UCD1, embedded in a
stellar stream around the spiral galaxy NGC 3628. The size and luminosity of UCD1 are similar to
ω Cen, the most luminous Milky Way globular cluster, which has long been suspected to
be the stripped remnant of an accreted dwarf galaxy. The object has a magnitude of
i = 19.3 mag (Li = 1.4 × 106
UCD1 is marginally resolved in our ground-based imaging, with a half-light radius of ~10 pc.
We measure an integrated brightness for the stellar stream of i = 13.1 mag, with
(g–i) = 1.0. This would correspond an accreted dwarf galaxy with an
approximate luminosity of Li ~ 4.1 × 108
L☉. Spectral analysis reveals that UCD1 has an age of 6.6 Gyr,
[Z/H] = –0.75, and [α/Fe] = –0.10. We propose that UCD1 is an
example of an ω Cen-like star cluster possibly forming from the nucleus of an
infalling dwarf galaxy, demonstrating that at least some of the massive star cluster population
may be created through tidal stripping.
VEGAS: A VST Early-type GAlaxy Survey. I. Presentation, wide-field surface photometry,
and substructures in NGC 4472
M. Capaccioli, M. Spavone, A. Grado, E. Iodice, L. Limatola, N.R. Napolitano, M. Cantiello, M. Paolillo, A.J. Romanowsky, D.A. Forbes, T.H. Puzia, G. Raimondo, P. Schipani
Astronomy & Astrophysics, Volume 581, September 2015, Article A10 (35pp)
We present the VST Early-type GAlaxy Survey (VEGAS), which is designed to
obtain deep multiband photometry in g,r,i,
of about one hundred nearby
galaxies down to 27.3, 26.8, and 26 mag/arcsec2 respectively,
using the ESO facility VST/OmegaCAM.
The goals of the survey are 1) to map the light distribution up to ten
effective radii, re; 2) to trace color gradients and
surface brightness fluctuation gradients out to a few re
for stellar population characterization; and 3) to obtain a full census of
the satellite systems (globular clusters and dwarf galaxies) out to 20% of
the galaxy virial radius. The external regions of galaxies retain signatures
of the formation and evolution mechanisms that shaped them, and the study of
nearby objects enables a detailed analysis of their morphology and
interaction features. To clarify the complex variety of formation mechanisms
of early-type galaxies (ETGs), wide and deep photometry is the primary
observational step, which at the moment has been pursued with only a few
dedicated programs. The VEGAS survey has been designated to provide these
data for a volume-limited sample with exceptional image quality.
In this commissioning photometric paper we illustrate the capabilities of
the survey using g- and i-band VST/OmegaCAM images of the
nearby galaxy NGC 4472 and of smaller ETGs in the surrounding field.
Our surface brightness profiles reach rather faint levels and agree
excellently well with previous literature. Genuine new results concern the
detection of an intracluster light tail in NGC 4472 and of various
substructures at increasing scales. We have also produced extended
(g–i) color profiles.
The VST/OmegaCAM data that we acquire in the context of the VEGAS survey
provide a detailed view of substructures in the optical emission from
extended galaxies, which can be as faint as a hundred times below the
The SLUGGS survey: globular cluster kinematics in a
'double sigma' galaxy – NGC 4473
A. Alabi, C. Foster, D.A. Forbes, A.J. Romanowsky, N. Pastorello,
J.P. Brodie, L.R. Spitler, J. Strader, C. Usher
Monthly Notices of the Royal Astronomical Society, Volume 452, Issue 2, 2015 September 11, pp. 2208–2219
NGC 4473 is a so-called double sigma (2σ) galaxy, i.e.
a galaxy with rare, double peaks in its 2D stellar velocity dispersion. Here, we present the globular cluster (GC) kinematics in NGC 4473 out to
~10 Re (effective radii) using data from combined Hubble
Space Telescope/Advanced Camera for Surveys and Subaru/Suprime-Cam imaging
and Keck/Deep Imaging Multi-Object Spectrograph. We find that the 2σ
nature of NGC 4473 persists up to 3 Re, though it becomes
misaligned to the photometric major axis. We also observe a significant offset
between the stellar and GC rotation amplitudes. This offset can be understood
as a co-addition of counter-rotating stars producing little net stellar
rotation. We identify a sharp radial transition in the GC kinematics at
~4 Re suggesting a well defined kinematically distinct halo.
In the inner region (<4 Re), the blue GCs rotate along the
photometric major axis, but in an opposite direction to the galaxy stars and
red GCs. In the outer region (>4 Re), the red GCs rotate in
an opposite direction compared to the inner region red GCs, along the
photometric major axis, while the blue GCs rotate along an axis intermediate
between the major and minor photometric axes. We also find a kinematically
distinct population of very red GCs in the inner region with elevated rotation
amplitude and velocity dispersion. The multiple kinematic components in NGC
4473 highlight the complex formation and evolutionary history of this
2σ galaxy, as well as a distinct transition between the inner
and outer components.
Hiding in plain sight: record-breaking compact stellar systems in the Sloan Digital Sky Survey
M.A. Sandoval, R.P. Vo, A.J. Romanowsky, J. Strader, J. Choi, Z.G. Jennings, C. Conroy, J.P. Brodie, C. Foster, A. Villaume, M.A. Norris, J. Janz, D.A. Forbes
The Astrophysical Journal Letters, Volume 808, Number 1, 2015 July 20, Article L32 (7pp)
Bad Astronomy |
AAS Nova |
Mercury News |
Motivated by the recent, serendipitous discovery of the densest known galaxy, M60-UCD1, we present
two initial findings from a follow-up search, using the Sloan Digital Sky Survey, Subaru/Suprime-Cam
and Hubble Space Telescope imaging, and SOuthern Astrophysics Research (SOAR)/Goodman spectroscopy.
The first object discovered, M59-UCD3, has a similar size to M60-UCD1 (half-light radius of rh ~ 20 pc) but
is 40% more luminous (MV ~ –14.6),
making it the new densest-known galaxy.
The second, M85-HCC1, has a size like a typical globular cluster
(GC; rh ~ 1.8 pc) but is much more luminous
(MV ~ –12.5).
This hypercompact cluster is by far the densest confirmed free-floating stellar system, and is equivalent to the densest known nuclear star clusters.
From spectroscopy, we find that both objects are
relatively young (~ 9 Gyr and ~ 3 Gyr, respectively), with metal-abundances that resemble those of galaxy centers.
Their host galaxies show clear signs of large-scale disturbances, and we conclude that these dense objects are the
remnant nuclei of recently accreted galaxies.
M59-UCD3 is an ideal target for follow-up with high-resolution imaging and spectroscopy to
search for an overweight central supermassive black hole as was discovered in M60-UCD1.
These findings also emphasize the potential value of
ultra-compact dwarfs and massive GCs as tracers of the assembly histories of galaxies.
The SLUGGS survey: inferring the formation epochs of metal-poor and metal-rich globular clusters
D.A. Forbes, N. Pastorello, A. Romanowsky, C. Usher, J.P. Brodie, J. Strader
Monthly Notices of the Royal Astronomical Society, Volume 452, Issue 1, 2015 September 01, pp. 1045–1051
We present a novel, observationally-based framework for the formation
epochs and sites of globular clusters (GCs) in a cosmological context. Measuring directly the mean ages of the metal-poor and metal-rich GC
subpopulations in our own Galaxy, and in other galaxies, is
observationally challenging. Here we apply an alternative approach
utilizing the property that the galaxy mass–metallicity relation
is a strong function of redshift (or look-back age) but is relatively
insensitive to galaxy mass for massive galaxies. Assuming that GCs
follow galaxy mass–metallicity relations that evolve with redshift,
one can estimate the mean formation epochs of the two GC subpopulations
by knowing their mean metallicities and the growth in host galaxy mass
with redshift. Recently, the SAGES Legacy Unifying Globulars and GalaxieS
(SLUGGS) survey has measured the spectroscopic metallicities for over
1000 GCs in a dozen massive early-type galaxies. Here we use these
measurements, and our new metallicity matching method, to infer a mean
age for metal-rich GCs of 11.5 Gyr (z = 2.9) and a range of
12.2–12.8 Gyr (4.8 < z < 5.9) for the metal-poor GCs,
depending on whether they mostly formed in accreted satellites or
in situ within the main host galaxy. We compare our values to
direct age measurements for Milky Way GCs and predictions from
cosmological models. Our findings suggest that reionization preceded
most GC formation, and that it is unlikely to be the cause of GC
bimodal metallicity distributions.
The megasecond Chandra X-ray visionary project observation of NGC 3115 (III): luminosity functions of LMXBs and dependence on stellar environments
D. Lin, J.A. Irwin, K.-W. Wong, Z.G. Jennings, J. Homan, A.J. Romanowsky, J. Strader, J.P. Brodie, G.R. Sivakoff, R.A. Remillard
The Astrophysical Journal, Volume 808, Number 1, 2015 July 20, Article 20 (11pp)
We studied the X-ray luminosity function (XLF) of low-mass X-ray binaries
(LMXBs) in the nearby lenticular galaxy NGC 3115, using the Megasecond
Chandra X-ray Visionary Project Observation. With a total exposure
time of ~1.1 Ms, we constructed the XLF down to a limiting luminosity of
~1036 erg s–1, which is much deeper than that
typically reached for other early-type galaxies. We found significant
flattening of the overall LMXB XLF from dN/dL ∝
L–2.2 ± 0.4 above 5.5 ×
1037 erg s–1 to dN/dL ∝
L–1.0 ± 0.1 below it, although we could
not rule out a fit with a higher break at ~1.6 × 1038
erg s–1. We also found evidence that the XLF of LMXBs
in globular clusters (GCs) is overall flatter than that of field LMXBs.
Thus, our results for this galaxy do not support the idea that all LMXBs
are formed in GCs. The XLF of field LMXBs seems to show spatial variation,
with the XLF in the inner region of the galaxy being flatter than that
in the outer region, probably due to contamination of LMXBs from
undetected and/or disrupted GCs in the inner region. The XLF in the outer
region is probably the XLF of primordial field LMXBs, exhibiting
dN/dL ∝ L–1.2 ± 0.1
up to a break close to the Eddington limit of neutron star LMXBs
(~1.7 × 1038 erg s–1). The break of
the GC LMXB XLF is lower, at ~1.1 × 1037 erg
s–1. We also confirm previous findings that the
metal-rich/red GCs are more likely to host LMXBs than the metal-poor/blue
GCs, which is more significant for more luminous LMXBs, and that more
massive GCs are more likely to host LMXBs.
The megasecond Chandra X-ray visionary project observation of NGC 3115 (II): properties of point sources
D. Lin, J.A. Irwin, K.-W. Wong, Z.G. Jennings, J. Homan, A.J. Romanowsky, J. Strader, G.R. Sivakoff, J.P. Brodie, R.A. Remillard
The Astrophysical Journal, Volume 808, Number 1, 2015 July 20, Article 19 (21pp)
We carried out an in-depth study of low-mass X-ray binaries (LMXBs)
detected in the nearby lenticular galaxy NGC 3115 using the Megasecond
Chandra X-ray Visionary Project observation (total exposure time
1.1 Ms). In total we found 136 candidate LMXBs in the field and 49 in globular clusters (GCs) above 2σ detection,
with 0.3–8 keV luminosity
LX ~ 1036–1039 erg
s–1. Other than 13 transient candidates, the sources
overall have less long-term variability at higher luminosity, at least
at LX >~ 1037 erg s–1.
In order to identify the nature and spectral
state of our sources, we compared their collective spectral properties
based on single-component models (a simple power law or a multicolor
disk) with the spectral evolution seen in representative Galactic LMXBs.
We found that in the LX versus photon index
LX versus disk temperature kTMCD
plots, most of our
sources fall on a narrow track in which the spectral shape hardens with
increasing luminosity below LX ~ 7 ×
1037 erg s–1, but is relatively
constant (ΓPL ~ 1.5 or kTMCD
~ 1.5 keV) above this luminosity, which is similar to the spectral
evolution of Galactic neutron star (NS) LMXBs in the soft state in the
Chandra bandpass. Therefore, we identified the track as the NS
LMXB soft-state track and suggested sources with LX
<~ 7 × 1037 erg s–1 as atolls in
the soft state and those with LX >~ 7 ×
1037 erg s–1 as Z sources. Ten other
sources (five are transients) displayed significantly softer spectra
and are probably black hole X-ray binaries in the thermal state. One of
them (persistent) is in a metal-poor GC.
VIMOS mosaic integral-field spectroscopy of the bulge and disc of the early-type galaxy NGC 4697
C. Spiniello, N.R. Napolitano, L. Coccato, V. Pota, A.J. Romanowsky, C. Tortora, G. Covone, M. Capaccioli
Monthly Notices of the Royal Astronomical Society, Volume 452, Issue 1, 2015 September 01, pp. 99–114
We present an integral-field study of the internal structure, kinematics and
stellar population of the almost edge-on, intermediate-luminosity
(L★) elliptical galaxy NGC 4697. We build extended
two-dimensional (2D) maps of the stellar kinematics and line strengths of the
galaxy up to ~0.7 effective radii (Reff) using a mosaic of
eight VIMOS (VIsible Multi-Objects Spectrograph, on the Very Large Telescope)
integral-field unit pointings. We find clear evidence for a rotation-supported
structure along the major axis from the 2D kinematical maps, confirming the
previous classification of this system as a 'fast rotator'. We study the
correlations between the third and fourth Gauss–Hermite moments of the
line-of-sight velocity distribution (LOSVD) h3 and
h4 with the rotation parameter (V/σ), and
compare our findings to hydrodynamical simulations. We find remarkable
similarities to predictions from gas-rich mergers. Based on photometry, we
perform a bulge/disc decomposition and study the stellar population properties
of the two components. The bulge and the disc show different stellar populations,
with the stars in the bulge being older
(agebulge = 13.5+1.4–1.4 Gyr,
agedisc = 10.5+1.6–2.0 Gyr) and more metal
poor ([M/H]bulge = –0.17+0.12–0.1,
[M/H]disc = –0.03+0.02–0.1).
The evidence of a later-formed, more metal-rich disc embedded in an older, more
metal poor bulge, together with the LOSVD structure, supports a mass assembly
scenario dominated by gas-rich minor mergers and possibly with a late gas-rich
major merger that left a previously rapidly rotating system unchanged. The bulge
and the disc do not show signs of different stellar initial mass function (IMF)
slopes, and both match well with a Milky Way-like IMF.
The SLUGGS survey: combining stellar and globular cluster
metallicities in the outer regions of early-type galaxies
N. Pastorello, D.A. Forbes, C. Usher, J.P. Brodie, A.J. Romanowsky,
J. Strader, L.R. Spitler, A.B. Alabi, C. Foster, Z.G. Jennings, S.S. Kartha,
Monthly Notices of the Royal Astronomical Society, Volume 451, Issue 3, 2015 August 11, pp. 2625–2639
The outer halo regions of early-type galaxies carry key information about their past
accretion history. However, spectroscopically probing the stellar component at such
galactocentric radii is still challenging. Using the DEep Imaging Multi-Object
Spectrograph on the Keck, we have been able to measure the metallicities of the
stellar and globular cluster components in 12 early-type galaxies out to more than
10 Re. We find similar metallicity gradients for the metal-poor
and metal-rich globular cluster subpopulations, suggesting a common formation
process for the two subpopulations. This is in conflict with most current
theoretical predictions, where the metal-poor globular clusters are thought to be
purely accreted and metal-rich globular clusters mostly formed in situ.
Moreover, we find that the globular cluster metallicity gradients show a trend with
galaxy mass, being steeper in lower mass galaxies than in higher mass galaxies.
This is similar to what we find for the outermost galaxy stars and suggests a more
active accretion history, with a larger role played by major mergers, in the most
massive galaxies. This conclusion is qualitatively consistent with expectations
from two-phase galaxy assembly models. Finally, we find that the small difference
in metallicity between galaxy stars and metal-rich globular clusters at 1
Re may correlate with galaxy mass. The origin of this difference
is not currently clear.
The SLUGGS survey: multipopulation dynamical modelling of the elliptical galaxy NGC 1407 from stars and globular clusters
V. Pota, A.J. Romanowsky, J.P. Brodie, J. Peñarrubia,
D.A. Forbes, N.R. Napolitano, C. Foster, M.G. Walker, J. Strader, J.C. Roediger
Monthly Notices of the Royal Astronomical Society, Volume 450, Issue 3, 2015 July 01, pp. 3345–3358
We perform in-depth dynamical modelling of the luminous and dark matter (DM)
content of the elliptical galaxy NGC 1407. Our strategy consists of solving the spherical Jeans equations for three independent dynamical tracers: stars,
blue globular clusters (GCs) and red GCs in a self-consistent manner. We adopt
a maximum-likelihood Markov Chain Monte Carlo fitting technique in the
attempt to constrain the inner slope of the DM density profile (the cusp/core
problem), and the stellar initial mass function (IMF) of the galaxy. We find
the inner logarithmic slope of the DM density profiles to be
γ = 0.6 ± 0.4, which is consistent with either a DM cusp
(γ = 1) or with a DM core (γ = 0). Our findings are
consistent with a Salpeter IMF, and marginally consistent with a Kroupa IMF.
We infer tangential orbits for the blue GCs, and radial anisotropy for red
GCs and stars. The modelling results are consistent with the virial
mass–concentration relation predicted by Λ cold dark matter (CDM)
simulations. The virial mass of NGC 1407 is log Mvir =
13.3 ± 0.2 M☉, whereas the stellar mass is
log M* = 11.8 ± 0.1 M☉. The overall
uncertainties on the mass of NGC 1407 are only 5 per cent at the projected
stellar effective radius. We attribute the disagreement between our results
and previous X-ray results to the gas not being in hydrostatic equilibrium in
the central regions of the galaxy. The halo of NGC 1407 is found be
DM-dominated, with a dynamical mass-to-light ratio of M/L =
M☉/L☉,B. However, this value can
be larger up to a factor of 3 depending on the assumed prior on the DM scale
Spectroscopic confirmation of the existence of large, diffuse galaxies in the Coma cluster
P.G. van Dokkum, A.J. Romanowsky, R. Abraham, J.P. Brodie, C. Conroy, M. Geha,
A. Merritt, A. Villaume, J. Zhang
The Astrophysical Journal Letters, Volume 804, Number 1, 2015 May 1, Article L26 (5pp)
Daily Mail |
We recently identified a population of low surface brightness objects in the field of the
z = 0.023 Coma cluster, using the Dragonfly Telephoto Array. Here we present Keck
spectroscopy of one of the largest of these "ultra-diffuse galaxies" (UDGs), confirming that it
is a member of the cluster. The galaxy has prominent absorption features, including the Ca II
H+K lines and the G-band, and no detected emission lines. Its radial velocity of
cz = 6280 ± 120 km s–1 is within the 1 σ velocity
dispersion of the Coma cluster. The galaxy has an effective radius of 4.3 ± 0.3 kpc and a
Sérsic index of 0.89 ± 0.06, as measured from Keck imaging. We find no indications
of tidal tails or other distortions, at least out to a radius of ~2 re.
We show that UDGs are located in a previously sparsely populated region of the
size–magnitude plane of quiescent stellar systems, as they are ~6 mag fainter than normal
early-type galaxies of the same size. It appears that the luminosity distribution of large
quiescent galaxies is not continuous, although this could largely be due to selection effects.
Dynamical measurements are needed to determine whether the dark matter halos of UDGs are
similar to those of galaxies with the same luminosity or to those of galaxies with the same size.
A SLUGGS and Gemini/GMOS combined study of the elliptical galaxy M60: wide-field photometry and kinematics of the globular cluster system
V. Pota, J.P. Brodie, T. Bridges, J. Strader, A.J. Romanowsky, A. Villaume,
Z. Jennings, F.R. Faifer, N. Pastorello, D.A. Forbes, A. Campbell, C. Usher, C. Foster,
L.R. Spiter, N. Caldwell, J.C. Forte, M.A. Norris, S.E. Zepf, M.A. Beasley,
K. Gebhardt, D.A. Hanes, R.M. Sharples, J.A. Arnold
Monthly Notices of the Royal Astronomical Society, Volume 450, Issue 2, 2015 June 21, pp. 1962–1983
We present new wide-field photometry and spectroscopy of the globular
clusters (GCs) around NGC 4649 (M60), the third brightest galaxy in
the Virgo cluster. Imaging of NGC 4649 was assembled from a recently
obtained Hubble Space Telescope/Advanced Camera for Surveys mosaic,
and new Subaru/Suprime-Cam and archival
Canada–France–Hawaii Telescope/MegaCam data. About 1200
sources were followed up spectroscopically using combined observations
from three multi-object spectrographs: Keck/Deep Imaging Multi-Object
Spectrograph, Gemini/Gemini Multi-Object Spectrograph and Multiple
Mirror Telescope/Hectospec. We confirm 431 unique GCs belonging to
NGC 4649, a factor of 3.5 larger than previous data sets and with a
factor of 3 improvement in velocity precision. We confirm significant
GC colour bimodality and find that the red GCs are more centrally
concentrated, while the blue GCs are more spatially extended. We infer
negative GC colour gradients in the innermost 20 kpc and flat
gradients out to large radii. Rotation is detected along the galaxy
major axis for all tracers: blue GCs, red GCs, galaxy stars and
planetary nebulae. We compare the observed properties of NGC 4649
with galaxy formation models. We find that formation via a major
merger between two gas-poor galaxies, followed by satellite accretion,
can consistently reproduce the observations of NGC 4649 at different
radii. We find no strong evidence to support an interaction between
NGC 4649 and the neighbouring spiral galaxy NGC 4647. We identify
interesting GC kinematic features in our data, such as
counter-rotating subgroups and bumpy kinematic profiles, which encode
more clues about the formation history of NGC 4649.
Small scatter and nearly-isothermal mass profiles to four half-light radii from two-dimensional stellar dynamics of early-type galaxies
M. Cappellari, A.J. Romanowsky, J.P. Brodie, D.A. Forbes, J. Strader, C. Foster, S.S. Kartha, N. Pastorello, V. Pota, L.R. Spitler, C. Usher, J.A. Arnold
The Astrophysical Journal Letters, Volume 804, Number 1, 2015 May 1, Article L21 (7pp)
We study the total mass-density profile for a sample of 14
fast-rotator early-type galaxies (stellar masses 10.2
<~ log M*/M☉ <~ 11.7).
We combine observations from the SLUGGS and ATLAS3D
surveys to map out the stellar kinematics in two dimensions,
out to a median radius for the sample of four half-light radii
Re (or 10 kpc)
and a maximum radius of 2.0–6.2 Re
(or 4–21 kpc).
We use axisymmetric dynamical models based on the Jeans equations,
which allow for a spatially varying anisotropy; employ quite general
profiles for the dark halos; and, in particular, do not place any
restrictions on the profile slope. This is made possible by the
availability of spatially extended two-dimensional kinematics. We
find that our relatively simple models provide a remarkably good
description of the observed kinematics. The resulting total density
profiles are well described by a nearly isothermal power law
ρtot(r) ∝ rγ
from Re/10 to at least 4 Re, the
largest average deviation being 11%. The average logarithmic slope is
〈γ〉 = 2.19 ± 0.03 with observed rms scatter of
just σγ = 0.11.
This scatter out to large radii,
where dark matter dominates, is as small as previously reported by
lensing studies around r ≈ Re/2,
where the stars dominate. Our bulge–halo conspiracy places much
tighter constraints on galaxy formation models. It illustrates the
power of two-dimensional stellar kinematics observations at large
radii. It is now important to test the generality of our results for
different galaxy types and larger samples.
How elevated is the dynamical-to-stellar mass ratio of the ultra-compact dwarf S999?
J. Janz, D.A. Forbes, M.A. Norris, J. Strader, S.J. Penny, M. Fagioli, A.J. Romanowsky
Monthly Notices of the Royal Astronomical Society, Volume 449, Issue 2, 2015 May 11, pp. 1716–1730
Here we present new Keck Echelle Spectrograph and Imager high-resolution spectroscopy
and deep archival Hubble Space Telescope/Advanced Camera for Surveys imaging for
S999, an ultracompact dwarf in the vicinity of M87, which was claimed to have an
extremely high dynamical-to-stellar mass ratio. Our data increase the total integration
times by a factor of 5 and 60 for spectroscopy and imaging, respectively. This allows us
to constrain the stellar population parameters for the first time (simple stellar
population equivalent age = 7.6+2.0–1.6 Gyr; [Z/H] =
–0.95+0.12–0.10; [α/Fe] =
0.34+0.10–0.12). Assuming a Kroupa stellar initial mass
function, the stellar population parameters and luminosity
(MF814W = –12.13 ± 0.06 mag) yield a stellar mass of
M* = 3.9+0.9–0.6
×106 M☉, which we also find to be consistent
with near-infrared data. Via mass modelling, with our new measurements of velocity
dispersion (σap = 27 ± 2 km s–1) and size
(Re = 20.9 ± 1.0 pc), we obtain an elevated dynamical-to-stellar
mass ratio Mdyn/M* = 8.2 (with a range 5.6 ≤
Mdyn/M* ≤ 11.2). Furthermore, we analyse the
surface brightness profile of S999, finding only a small excess of light in the outer
parts with respect to the fitted Sérsic profile, and a positive colour gradient.
Taken together these observations suggest that S999 is the remnant of a much larger
galaxy that has been tidally stripped. If so, the observed elevated mass ratio may be
caused by mechanisms related to the stripping process: the existence of a massive
central black hole or internal kinematics that are out of equilibrium due to the
stripping event. Given the observed dynamical-to-stellar mass ratio we suggest that S999
is an ideal candidate to search for the presence of an overly massive central black hole.
VEGAS-SSS. A VST early-type galaxy survey: analysis of small stellar systems. Testing the methodology on the globular cluster system in NGC 3115
M. Cantiello, M. Capaccioli, N. Napolitano, A. Grado, L. Limatola, M. Paolillo, E. Iodice,
A.J. Romanowsky, D.A. Forbes, G. Raimondo, M. Spavone, F. La Barbera, T.H. Puzia, P. Schipani
Astronomy & Astrophysics, Volume 576, April 2015, Article A14 (19pp)
We present a study of globular clusters (GCs) and other small stellar systems (SSSs) in the field of
NGC 3115, observed as part of the ongoing wide-field imaging survey VEGAS, carried out with the 2.6 m
VST telescope. We used deep g and i observations of NGC 3115, a well-studied lenticular galaxy that is
covered excellently well in the scientific literature. This is fundamental to test the methodologies,
verify the results, and probe the capabilities of the VEGAS-SSS. Leveraging the large field of view of
the VST allowed us to accurately study the distribution and properties of SSSs as a function of
galactocentric distance, well beyond ~20 galaxy effective radii, in a way that is rarely possible. Our
analysis of colors, magnitudes, and sizes of SSS candidates confirms the results from existing studies,
some of which were carried out with 8–10 m class telescopes, and further extends them to
previously unreached galactocentric distances with similar accuracy. In particular, we find a color
bimodality for the GC population and a de Vaucouleurs r1/4 profile for the surface density of GCs
similar to the galaxy light profile. The radial color gradient of blue and red GCs previously found,
for instance, by the SLUGGS survey with Subaru and Keck data, is further extended out to the largest
galactocentric radii inspected, ~65 kpc. In addition, the surface density profiles of blue and red GCs
taken separately are well approximated by a r1/4 density profile, with the fraction of blue GCs being
slightly larger at larger radii. We do not find hints of a trend for the red GC subpopulation and for
the GC turnover magnitude to vary with radius, but we observe a ~0.2 mag difference in the turnover
magnitude of the blue and red GC subpopulations. Finally, from inspecting SSS sizes and colors, we
obtain a list of ultracompact dwarf galaxies and GC candidates suitable for future spectroscopic
follow-up. In conclusion, our study shows i) the reliability of the methodologies developed to study
SSSs in the field of bright early-type galaxies; and ii) the great potential of the VEGAS survey to
produce original results on SSSs science, mainly thanks to the wide-field imaging adopted.
Detection of a distinct metal-poor stellar halo in the early-type galaxy NGC 3115
M.B. Peacock, J. Strader, A.J. Romanowsky, J.P. Brodie
The Astrophysical Journal, Volume 800, Number 1, 2015 February 10, Article 13 (17pp)
We present the resolved stellar populations in the inner and outer halo of the
nearby lenticular galaxy NGC 3115. Using deep Hubble Space Telescope observations,
we analyze stars 2 mag fainter than the tip of the red giant branch (TRGB).
We study three fields along the minor axis of this galaxy, 19, 37, and 54 kpc from
its center – corresponding to 7, 14, and 21 effective radii (re).
Even at these large galactocentric distances, all of the fields are dominated by a
relatively enriched population, with the main peak in the metallicity distribution
decreasing with radius from [Z/H] ~ –0.5 to –0.65. The fraction of
metal-poor stars ([Z/H] < –0.95) increases from 17% at 16–37 kpc to 28% at
~54 kpc. We observe a distinct low-metallicity population (peaked at
[Z/H] ~ –1.3 and with total mass 2×1010 M☉
~ 14% of the galaxy's
stellar mass) and argue that this represents the detection of an underlying
low-metallicity stellar halo. Such halos are generally predicted by galaxy
formation theories and have been observed in several late-type galaxies, including
the Milky Way and M31. The metallicity and spatial distribution of the stellar halo
of NGC 3115 are consistent with the galaxy's globular cluster system, which has a
similar low-metallicity population that becomes dominant at these large radii.
This finding supports the use of globular clusters as bright chemodynamical tracers
of galaxy halos. These data also allow us to make a precise measurement of the
magnitude of the TRGB, from which we derive a distance modulus of NGC 3115 of
30.05 ± 0.05 ± 0.10sys (10.2 ± 0.2 ± 0.5sys Mpc).
The SLUGGS survey: globular cluster stellar population trends from weak absorption lines in stacked spectra
C. Usher, D.A. Forbes, J.P. Brodie, A.J. Romanowsky, J. Strader, C. Conroy, C. Foster, N. Pastorello, V. Pota, J.A. Arnold
Monthly Notices of the Royal Astronomical Society, Volume 446, Issue 1, 2015 January 1, pp. 369–390
As part of the SAGES Legacy Unifying Globulars and GalaxieS (SLUGGS) survey,
we stack 1137 Keck DEIMOS (Deep Imaging Multi-Object Spectrograph) spectra of
globular clusters from 10 galaxies to study their stellar populations in detail.
The stacked spectra have median signal-to-noise ratios of ~90 Å–1.
Besides the calcium triplet, we study weaker sodium, magnesium, titanium and
iron lines as well as the Hα and higher order Paschen hydrogen lines.
In general, the stacked spectra are consistent with old ages and a Milky
Way-like initial mass function. However, we see different metal line index
strengths at fixed colour and magnitude, and differences in the calcium
triplet-colour relation from galaxy to galaxy. We interpret this as strong
evidence for variations in the globular cluster colour–metallicity relation
between galaxies. Two possible explanations for the colour–metallicity relation
variations are that the average ages of globular clusters vary from galaxy to
galaxy or that the average abundances of light elements (i.e. He, C, N and O)
differ between galaxies. Stacking spectra by magnitude, we see that the colours
become redder and metal line indices stronger with brighter magnitudes. These
trends are consistent with the previously reported 'blue tilts' being
The SAGES Legacy Unifying Globulars and GalaxieS survey (SLUGGS): sample definition, methods, and initial results
J.P. Brodie, A.J. Romanowsky, J. Strader, D.A. Forbes, C. Foster, Z.G. Jennings, N. Pastorello, V. Pota, C. Usher, C. Blom, J. Kader, J.C. Roediger, L.R. Spitler, A. Villaume, J.A. Arnold, S.S. Kartha, K.A. Woodley
The Astrophysical Journal, Volume 796, Number 1, 2014 November 20, Article 52 (25pp)
We introduce and provide the scientific motivation for a wide-field photometric and spectroscopic chemodynamical survey of nearby early-type galaxies (ETGs) and their globular cluster (GC) systems. The SAGES Legacy Unifying Globulars and GalaxieS (SLUGGS) survey is being carried out primarily with Subaru/Suprime-Cam and Keck/DEIMOS. The former provides deep gri imaging over a 900 arcmin2 field-of-view to characterize GC and host galaxy colors and spatial distributions, and to identify spectroscopic targets. The NIR Ca II triplet provides GC line-of-sight velocities and metallicities out to typically ~8 Re, and to ~15 Re in some cases. New techniques to extract integrated stellar kinematics and metallicities to large radii (~2–3 Re) are used in concert with GC data to create two-dimensional (2D) velocity and metallicity maps for comparison with simulations of galaxy formation. The advantages of SLUGGS compared with other, complementary, 2D-chemodynamical surveys are its superior velocity resolution, radial extent, and multiple halo tracers. We describe the sample of 25 nearby ETGs, the selection criteria for galaxies and GCs, the observing strategies, the data reduction techniques, and modeling methods. The survey observations are nearly complete and more than 30 papers have so far been published using SLUGGS data. Here we summarize some initial results, including signatures of two-phase galaxy assembly, evidence for GC metallicity bimodality, and a novel framework for the formation of extended star clusters and ultracompact dwarfs. An integrated overview of current chemodynamical constraints on GC systems points to separate, in situ formation modes at high redshifts for metal-poor and metal-rich GCs.
Evolution of central dark matter of early-type galaxies up to z ~ 0.8
C. Tortora, N.R. Napolitano, R.P. Saglia, A.J. Romanowsky, G. Covone, M. Capaccioli
Monthly Notices of the Royal Astronomical Society, Volume 445, Issue 1, 2014 November 21, pp. 162–174
We investigate the evolution of dark and luminous matter in the central regions of early-type galaxies up to z ~ 0.8.
We use a spectroscopically selected sample of 154 cluster and field galaxies from the ESO Distant Clusters Survey (EDisCS),
covering a wide range in redshifts (z ~ 0.4–0.8),
stellar masses (log M★/M☉ ~ 10.5–11.5 dex)
and velocity dispersions (σ★ ~ 100–300 km s–1).
We obtain central dark matter (DM) fractions by determining the dynamical masses from Jeans modelling of galaxy aperture velocity dispersions and the M★ from galaxy colours, and compare the results with local samples.
We discuss how the correlations of central DM with galaxy size (i.e. the effective radius, Re),
M★ and σ★ evolve as a function of redshift,
finding clear indications that local galaxies are, on average, more DM dominated than their counterparts at larger redshift.
This DM fraction evolution with z can be only partially interpreted as a consequence of the size–redshift evolution. We discuss our results within galaxy formation scenarios, and conclude that the growth in size and DM content which we measure within the last 7 Gyr is incompatible with passive evolution, while it is well reproduced in the multiple minor merger scenario.
We also discuss the impact of the initial mass function (IMF) on our DM inferences and argue that this can be non-universal with the look-back time.
In particular, we find that the Salpeter IMF can be better accommodated by low-redshift systems, while producing stellar masses at high z
which are unphysically larger than the estimated dynamical masses (particularly for lower σ★ systems).
Systematic variations of central mass density slopes in early-type galaxies
C. Tortora, F. La Barbera, N.R. Napolitano, A.J. Romanowsky, I. Ferreras, R.R. de Carvalho
Monthly Notices of the Royal Astronomical Society, Volume 445, Issue 1, 2014 November 21, pp. 115–127
We study the total density distribution in the central regions (≲1 effective radius,
Re) of early-type galaxies (ETGs), using data from SPIDER and ATLAS3D.
Our analysis extends the range of galaxy stellar mass
(M★) probed by gravitational lensing, down to ~1010 M☉.
We model each galaxy with two components (dark matter halo + stars), exploring different assumptions for the dark matter halo profile (i.e. NFW, NFW-contracted, and Burkert profiles),
and leaving stellar mass-to-light (M★/L) ratios as free fitting parameters to the data.
For all plausible halo models, the best-fitting M★/L, normalized to that for a Chabrier initial mass function, increases systematically with galaxy size and mass.
For an NFW profile, the slope of the total mass profile is non-universal, independently of several ingredients in the modelling (e.g. halo contraction, anisotropy, and rotation velocity in ETGs).
For the most massive (M★ ~ 1011.5 M☉)
or largest (Re ~ 15 kpc) ETGs, the profile is isothermal in the central regions
(~ Re/2), while for the low-mass (M★ ~ 1010.2 M☉)
or smallest (Re ~ 0.5 kpc) systems, the profile is steeper than isothermal, with slopes similar to those for a constant-M/L profile.
For a steeper concentration–mass relation than that expected from simulations, the correlation of density slope with galaxy mass tends to flatten, while correlations with Re and velocity dispersions are more robust.
Our results clearly point to a 'non-homology' in the total mass distribution of ETGs, which simulations of galaxy formation suggest may be related to a varying role of dissipation with galaxy mass.
The AIMSS Project – II. Dynamical-to-stellar mass ratios
across the star cluster–galaxy divide
D.A. Forbes, M.A. Norris, J. Strader, A.J. Romanowsky,
V. Pota, S.J. Kannappan, J.P. Brodie, A. Huxor
Monthly Notices of the Royal Astronomical Society, Volume 444, Issue 4, 2014 November 11, pp. 2993–3003
The previously clear division between small galaxies and massive star clusters is now occupied by objects called ultra-compact dwarfs (UCDs) and compact ellipticals (cEs). Here we combine a sample of UCDs and cEs with velocity dispersions from the AIMSS project with literature data to explore their dynamical-to-stellar mass ratios.
We confirm that the mass ratios of many UCDs in the stellar mass range
106–109 M☉ are systematically higher than those for globular clusters which have mass ratios near unity.
However, at the very highest masses in our sample, i.e.
we find that cE galaxies also have mass ratios of close to unity, indicating their central regions are mostly composed of stars.
Suggested explanations for the elevated mass ratios of UCDs have included a variable IMF, a central black hole, and the presence of dark matter. Here we present another possible explanation, i.e. tidal stripping. Under various assumptions, we find that the apparent variation in the mass ratio with stellar mass and stellar density can be qualitatively reproduced by published tidal stripping simulations of a dwarf elliptical galaxy.
In the early stages of the stripping process the galaxy is unlikely to be in virial equilibrium. At late stages, the final remnant resembles the properties of ~107
M☉ UCDs. Finally, we discuss the need for more detailed realistic modelling of tidal stripping over a wider range of parameter space, and observations to further test the stripping hypothesis.
Simulating multiple merger pathways to the central kinematics of early-type galaxies
C.E. Moody, A.J. Romanowsky, T.J. Cox, G.S. Novak, J.R. Primack
Monthly Notices of the Royal Astronomical Society, Volume 444, Issue 2, 2014 October 21, pp. 1475–1485
Two-dimensional integral field surveys such as ATLAS3D are producing rich observational data sets yielding insights into galaxy formation. These new kinematic observations have highlighted the need to understand the evolutionary mechanisms leading to a spectrum of fast rotators and slow rotators in early-type galaxies. We address the formation of slow and fast rotators through a series of controlled, comprehensive hydrodynamical simulations, sampling idealized galaxy merger scenarios constructed from model spiral galaxies. Idealized and controlled simulations of this sort complement the more 'realistic' cosmological simulations by isolating and analysing the effects of specific parameters, as we do in this paper. We
recreate minor and major binary mergers, binary merger trees with multiple progenitors, and multiple sequential mergers. Within each of these categories of formation history, we correlate progenitor gas fraction, mass ratio, orbital pericentre, orbital ellipticity, and spin with remnant kinematic properties. We create kinematic profiles of these 95 simulations comparable to ATLAS3D data. By constructing remnant profiles of the projected specific angular momentum
(λR = <R|V|> / <R (V2+σ2)1/2>),
triaxiality, and measuring the incidences of kinematic twists and kinematically decoupled cores, we distinguish between varying formation scenarios. We find that binary mergers nearly always form fast rotators. Slow rotators can be formed from zero initial angular momentum configurations and gas-poor mergers, but are not as round as the ATLAS3D galaxies. Remnants of binary merger trees are triaxial slow rotators. Sequential mergers form round slow rotators that most resemble the ATLAS3D rotators.
The SLUGGS Survey: wide-field stellar kinematics of early-type galaxies
J.A. Arnold, A.J. Romanowsky, J.P. Brodie, D.A. Forbes, J. Strader, L.R. Spitler, C. Foster, C. Blom, S.S. Kartha, N. Pastorello, V. Pota, C. Usher, K.A. Woodley
The Astrophysical Journal, Volume 791, Number 2, 2014 August 20, Article 80 (27pp)
We present stellar kinematics of 22 nearby early-type galaxies (ETGs), based on two-dimensional (2D) absorption
line stellar spectroscopy out to ~2–4 Re (effective radii), as part of the ongoing SLUGGS Survey. The galaxies
span a factor of 20 in intrinsic luminosity, as well as a full range of environment and ETG morphology. Our data
consist of good velocity resolution (σinst ~ 25 km s–1) integrated stellar-light spectra extracted from the individual
slitlets of custom made Keck/DEIMOS slitmasks. We extract stellar kinematics measurements
(V, σ, h3, and h4)
for each galaxy. Combining with literature values from smaller radii, we present 2D spatially resolved maps of the
large-scale kinematic structure in each galaxy. We find that the kinematic homogeneity found inside 1 Re often
breaks down at larger radii, where a variety of kinematic behaviors are observed. While central slow rotators remain
slowly rotating in their halos, central fast rotators show more diversity, ranging from rapidly increasing to rapidly
declining specific angular momentum profiles in the outer regions. There are indications that the outer trends
depend on morphological type, raising questions about the proposed unification of the elliptical and lenticular (S0)
galaxy families in the ATLAS3D survey. Several galaxies in our sample show multiple lines of evidence for distinct
disk components embedded in more slowly rotating spheroids, and we suggest a joint photometric–kinematic
approach for robust bulge–disk decomposition. Our observational results appear generally consistent with a picture
of two-phase (in-situ plus accretion) galaxy formation.
The AIMSS Project – I. Bridging the star cluster–galaxy divide
M.A. Norris, S.J. Kannappan, D.A. Forbes, A.J. Romanowsky, J.P. Brodie,
F.R. Faifer, A. Huxor, C. Maraston, A.J. Moffett, S.J. Penny, V. Pota,
A. Smith-Castelli, J. Strader, D. Bradley, K. Eckert, D. Fohring,
J. McBride, D. Stark, O. Vaduvescu
Monthly Notices of the Royal Astronomical Society, Volume 443, Issue 2, 2014 September 11, pp. 1151–1172
We describe the structural and kinematic properties of the first compact stellar systems discovered by the AIMSS project. These spectroscopically confirmed objects have sizes
(~6 < Re [pc] < 500) and masses
(~2×106 < M*/M☉ < 6×109) spanning the range of massive globular clusters (GCs), ultra compact dwarfs (UCDs) and compact elliptical galaxies (cEs), completely filling the gap between star clusters and galaxies.
Several objects are close analogues to the prototypical cE, M32. These objects, which are more massive than previously discovered UCDs of the same size, further call into question the existence of a tight mass-size trend for compact stellar systems, while simultaneously strengthening the case for a universal "zone of avoidance" for dynamically hot stellar systems in the mass-size plane.
Overall, we argue that there are two classes of compact stellar systems: 1) massive star clusters and 2) a population closely related to galaxies. Our data provide indications for a further division of the galaxy-type UCD/cE population into two groups, one population that we associate with objects formed by the stripping of nucleated dwarf galaxies, and a second population that formed through the stripping of bulged galaxies or are lower-mass analogues of classical ellipticals. We find compact stellar systems around galaxies in low to high density environments, demonstrating that the physical processes responsible for forming them do not only operate in the densest clusters.
Kinematics and simulations of the stellar stream in the halo of the Umbrella Galaxy
C. Foster, H. Lux, A.J. Romanowsky, D. Martínez-Delgado, S. Zibetti, J.A. Arnold, J.P. Brodie, R. Ciardullo, R.J. GaBany, M.R. Merrifield, N. Singh, J. Strader
Monthly Notices of the Royal Astronomical Society, Volume 442, Issue 4, 2014 August 11, pp. 3544–3564
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We study the dynamics of faint stellar substructures around the Umbrella Galaxy,
NGC 4651, which hosts a dramatic system of streams and shells formed through
the tidal disruption of a nucleated dwarf elliptical galaxy. We elucidate the basic characteristics of the system (colours, luminosities, stellar masses) using multi-band Subaru/Suprime-Cam images. The implied stellar mass-ratio of the ongoing merger event is ~1:50. We identify candidate kinematic tracers (globular clusters, planetary nebulae, HII regions), and follow up a subset with Keck/DEIMOS spectroscopy to obtain velocities. We find 15 of the tracers are likely associated with halo substructures, including the probable stream progenitor nucleus. These objects delineate a kinematically cold feature in position–velocity phase space. We model the stream using single test-particle orbits, plus a rescaled pre-existing N-body simulation. We infer a very eccentric orbit with a period of ~0.35 Gyr and turning points at ~2–4 and ~40 kpc, implying a recent passage of the satellite through the disc, which may have provoked the visible disturbances in the host galaxy. This work confirms that the kinematics of low surface brightness substructures can be recovered and modelled using discrete
tracers – a breakthrough that opens up a fresh avenue for unraveling the detailed
physics of minor merging.
Dynamical models of elliptical galaxies – II. M87 and its globular clusters
A. Agnello, N.W. Evans, A.J. Romanowsky, J.P. Brodie
Monthly Notices of the Royal Astronomical Society, Volume 442, Issue 4, 2014 August 11, pp. 3299–3314
We study the globular cluster (GC) system of the nearby elliptical galaxy M87 using the newly available data set with accurate kinematics provided by Strader et al. We find evidence for three distinct subpopulations of GCs in terms of colours, kinematics and radial profiles. A decomposition into three populations - blue, intermediate and red GCs - is statistically preferred to one with two or four populations. The existence of three components has been suggested before, but here we are able to identify them robustly and relate them to the stellar profile.
We exploit the subpopulations to derive dynamical constraints on the mass and dark matter (DM) content of M87 out to ~100 kpc. We deploy a class of global mass estimators, developed in Paper I, obtaining mass measurements at different locations. The DM fraction in M87 changes from ≈ 0.2 at the effective radius of the stellar light
(0.02° or 6 kpc)
to ≈ 0.95 at the distance probed by the most extended, blue GCs
(0.47° or 135 kpc).
We complete this analysis with virial decompositions, in which the dynamical model is used to produce velocity dispersions, which in turn are used to separate the GC populations. This ensures that the three subpopulations are simultaneously consistent with the same underlying mass profile. These yield the luminous mass as
and the DM within 135 kpc as 8.0+1.0–4.0×1012 M☉.
The inner DM density behaves as ρ ~ r–γ
with γ ≈ 1.6. This is steeper than the cosmologically preferred cusp
of ρ ~ r–1 and may provide evidence of DM contraction. Finally, we combine the GC separation into three subpopulations with the Jeans equations, obtaining information on the orbital structure of the GC system. The centrally concentrated red GCs exhibit tangential anisotropy, consistent with the depletion of radial orbits by tidal shredding. The most extended blue GCs have an isotropic velocity-dispersion tensor in the central parts, which becomes more tangential moving outwards, consistent with adiabatic contraction scenarios of the DM halo.
Dynamical models of elliptical galaxies – I. Simple methods
A. Agnello, N.W. Evans, A.J. Romanowsky
Monthly Notices of the Royal Astronomical Society, Volume 442, Issue 4, 2014 August 11, pp. 3284–3298
We study dynamical models for elliptical galaxies, deriving the projected kinematic profiles in a form that is valid for general surface brightness laws and (spherical) total mass profiles, without the need for any explicit deprojection. We provide accurate approximations of the line of sight and aperture-averaged velocity dispersion profiles for galaxies with total mass density profiles with slope near –2 and with modest velocity anisotropy using only single or double integrals, respectively.
This is already sufficient to recover many of the kinematic properties of nearby ellipticals. As an application, we provide two different sets of mass estimators for elliptical galaxies, based on either the velocity dispersion at a location at or near the effective radius, or the aperture-averaged velocity dispersion. In the large aperture (virial) limit, mass estimators are naturally independent of anisotropy. The spherical mass enclosed within the effective radius
can be estimated as 2.4 Re<σp2>/G,
where <σp2> is the average of the squared velocity dispersion over a finite aperture. This formula does not depend on assumptions such as mass-follows-light, and is a compromise between the cases of small and large aperture sizes. Its general agreement with results from other methods in the literature makes it a reliable means to infer masses in the absence of detailed kinematic information. If on the other hand the velocity dispersion profile is available, tight mass estimates can be found that are independent of the mass-model and anisotropy profile.
In particular, for a de Vaucouleurs surface brightness, the velocity dispersion measured
at ≈ 1 Re yields a tight mass estimate (with 10 per cent accuracy)
at ≈ 3 Re that is independent of the mass model and the anisotropy profile.
This allows us to probe the importance of dark matter at radii where it dominates the mass budget of galaxies. Explicit formulae are given for small anisotropy, large radii and/or power-law total densities. Motivated by recent observational claims, we also discuss the issue of weak homology of elliptical galaxies, emphasizing the interplay between morphology and orbital structure.
The SLUGGS Survey: HST/ACS mosaic imaging of the NGC 3115 globular cluster system
Z.G. Jennings, J. Strader, A.J. Romanowsky, J.P. Brodie, J.A. Arnold, D. Lin, J.A. Irwin,
G.R. Sivakoff, K.-W. Wong
The Astronomical Journal, Volume 148, Number 2, 2014 August, Article 32 (16pp)
We present Hubble Space Telescope/Advanced Camera for Surveys (HST/ACS)
g and z photometry and half-light radii Rh measurements
of 360 globular cluster (GC) candidates around the nearby S0 galaxy NGC 3115.
We also include Subaru/Suprime-Cam g, r, and i photometry of 421 additional candidates.
The well-established color bimodality of the GC system is obvious in the HST/ACS
photometry. We find evidence for a "blue tilt" in the blue GC subpopulation,
wherein the GCs in the blue subpopulation get redder as luminosity increases,
indicative of a mass-metallicity relationship. We find a color gradient in both the
red and blue subpopulations, with each group of clusters becoming bluer at larger
distances from NGC 3115. The gradient is of similar strength in both subpopulations,
but is monotonic and more significant for the blue clusters. On average, the blue
clusters have ~10% larger Rh than the red clusters. This average
difference is less than is typically observed for early-type galaxies but does match
that measured in the literature for the Sombrero Galaxy (M104), suggesting that
morphology and inclination may affect the measured size difference between the red
and blue clusters. However, the scatter on the Rh measurements is large. We also
identify 31 clusters more extended than typical GCs, which we term ultra-compact
dwarf (UCD) candidates. Many of these objects are actually considerably fainter
than typical UCDs. While it is likely that a significant number will be background
contaminants, six of these UCD candidates are spectroscopically confirmed as NGC 3115
members. To explore the prevalence of low-mass X-ray binaries in the GC system, we
match our ACS and Suprime-Cam detections to corresponding Chandra X-ray sources. We
identify 45 X-ray-GC matches: 16 among the blue subpopulation and 29 among the red
subpopulation. These X-ray/GC coincidence fractions are larger than is typical for
most GC systems, probably due to the increased depth of the X-ray data compared to
previous studies of GC systems.
The SLUGGS survey: exploring the metallicity gradients of nearby early-type galaxies to large radii
N. Pastorello, D.A. Forbes, C. Foster, J.P. Brodie, C. Usher, A.J. Romanowsky,
J. Strader, J.A. Arnold
Monthly Notices of the Royal Astronomical Society, Volume 442, Issue 2, 2014 August 1, pp. 1003–1039
Stellar metallicity gradients in the outer regions of galaxies are a critical tool for disentangling the contributions of in situ and ex situ
formed stars. In the two-phase galaxy formation scenario, the initial gas collapse creates steep metallicity gradients, while the accretion of stars formed in satellites tends to flatten these gradients in the outskirts, particularly for massive galaxies. This work presents the first compilation of extended metallicity profiles over a wide range of galaxy mass. We use the DEep Imaging Multi-Object Spectrograph spectrograph on the Keck telescope in multislit mode to obtain radial stellar metallicity profiles for 22 nearby early-type galaxies. From the calcium triplet lines in the near-infrared, we measure the metallicity of the starlight up to 3 effective radii. We find a relation between the outer metallicity gradient and galaxy mass, in the sense that lower mass systems show steeper metallicity gradients than more massive galaxies. This result is consistent with a picture in which the ratio of ex situ to in situ
formed stars is lower in less massive galaxies as a consequence of the smaller contribution by accretion. In addition, we infer a correlation between the strength of the calcium triplet feature in the near-infrared and the stellar initial mass function slope that is consistent with recent models in the literature.
A globular cluster toward M87 with a radial velocity < –1000 km s–1: the first hypervelocity cluster
N. Caldwell, J. Strader, A.J. Romanowsky, J.P. Brodie, B. Moore, J. Diemand, D. Martizzi
The Astrophysical Journal Letters, Volume 787, Number 1, 2014 May 20, Article L11 (5pp)
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We report the discovery of an object near M87 in the Virgo Cluster with an extraordinary blueshift of –1025 km s–1,
offset from the systemic velocity by >2300 km s–1.
Evaluation of photometric and spectroscopic data provides strong evidence that this object is a distant massive globular cluster, which we call HVGC-1 in analogy to Galactic hypervelocity stars. We consider but disfavor more exotic interpretations, such as a system of stars bound to a recoiling black hole. The odds of observing an outlier as extreme as HVGC-1 in a virialized distribution of intracluster objects are small; it appears more likely that the cluster was (or is being) ejected from Virgo following a three-body interaction. The nature of the interaction is unclear, and could involve either a subhalo or a binary supermassive black hole at the center of M87.
Ultracompact dwarfs in the Perseus Cluster: UCD formation via tidal stripping
S.J. Penny, D.A. Forbes, J. Strader, C. Usher, J.P. Brodie, A.J. Romanowsky
Monthly Notices of the Royal Astronomical Society, Volume 439, Issue 4, 2014 April 21, pp. 3808–3816
We present the results of a Keck/DEIMOS survey of Ultracompact dwarfs (UCDs) in the
Perseus Cluster core. We confirm cluster membership for 14 UCDs, with radial velocities
~5300 km s–1. Two of these confirmed that Perseus UCDs have extremely
blue colours (B – R < 0.6 mag), reside in star-forming filaments
surrounding NGC 1275, and have likely formed as massive star clusters in the last
~100 Myr. We also measure a central velocity dispersion of a third, UCD13
(σ0 = 38 ± 8 km s–1), the most extended
UCD in our sample. We determine it to have radius Re = 85 ± 1.1 pc, a
dynamical mass of (2.3 ± 0.8) × 108 M☉, and a
metallicity [Z/H] = –0.52+0.33–0.29 dex.
UCD13 and the cluster's central galaxy, NGC 1275, have a projected separation of
30 kpc and a radial velocity difference of ~20 km s–1. Based on
its size, red colour, internal velocity dispersion, dynamical mass, metallicity,
and proximity to NGC 1275, we argue that UCD13 is likely the remnant nucleus of
a tidally stripped dwarf elliptical (dE), with this progenitor dE having
MB ≈ –16 mag and mass ~109 M☉.
The SLUGGS Survey: new evidence for a tidal interaction between the early-type
galaxies NGC 4365 and NGC 4342
C. Blom, D.A. Forbes, C. Foster, A.J. Romanowsky, J.P. Brodie
Monthly Notices of the Royal Astronomical Society, Volume 439, Issue 3, 2014 Apr 11, pp. 2420–2431
We present new imaging and spectral data for globular clusters (GCs) around NGC 4365 and NGC 4342. NGC 4342 is a compact, X-ray luminous S0 galaxy with an unusually massive central black hole. NGC 4365 is another atypical galaxy that dominates the W' group of which NGC 4342 is a member. Using imaging from the MegaCam instrument on the Canada–France–Hawaii Telescope we identify a stream of GCs between the two galaxies and extending beyond NGC 4342. The stream of GCs is spatially coincident with a stream/plume of stars previously identified. We find that the photometric colours of the stream GCs match those associated with NGC 4342, and that the recession velocity of the combined GCs from the stream and NGC 4342 matches the recession velocity for NGC 4342 itself. These results suggest that NGC 4342 is being stripped of GCs (and stars) as it undergoes a tidal interaction with the nearby elliptical galaxy NGC 4365. We compare NGC 4342 to two well-known, tidally stripped galaxies (M32 and NGC 4486B) and find various similarities. We also discuss previous claims by Bogdán et al. that NGC 4342 cannot be undergoing significant tidal stripping because it hosts a large dark matter halo.
The SLUGGS survey: breaking degeneracies between dark matter, anisotropy and the IMF using globular cluster subpopulations in the giant elliptical NGC 5846
N.R. Napolitano, V. Pota, A.J. Romanowsky, D.A. Forbes, J.P. Brodie, C. Foster
Monthly Notices of the Royal Astronomical Society, Volume 439, Issue 1, 2014 March 21, pp. 659–672
We study the mass and anisotropy distribution of the giant elliptical galaxy NGC 5846 using stars, as well as the red and blue globular cluster (GC) subpopulations. We break degeneracies in the dynamical models by taking advantage of the different phase space distributions of the two GC subpopulations to unambiguously constrain the mass of the galaxy and the anisotropy of the GC system. Red GCs show the same spatial distribution and behaviour as the starlight, whereas blue GCs have a shallower density profile, a larger velocity dispersion and a lower kurtosis, all of which suggest a different orbital distribution. We use a dispersion–kurtosis Jeans analysis and find that the solutions of separate analyses for the two GC subpopulations overlap in the halo parameter space. The solution converges on a massive dark matter halo, consistent with expectations from Λ cold dark matter (ΛCDM) and 7-year Wilkinson Microwave Anisotropy Probe (WMAP7) cosmology in terms of virial mass (log MDM ~ 13.3 M☉) and concentration (cvir ~ 8).
This is the first such analysis that solves the dynamics of the different GC subpopulations in a self-consistent manner. Our method improves the uncertainties on the halo parameter determination by a factor of 2 and opens new avenues for the use of elliptical galaxy dynamics as tests of predictions from cosmological simulations. The implied stellar mass-to-light ratio derived from the dynamical modelling is fully consistent with a Salpeter initial mass function (IMF) and rules out a bottom light IMF. The different GC subpopulations show markedly distinct orbital distributions at large radii, with red GCs having an anisotropy parameter β ~ 0.4 outside ~ 3 Re (Re is the effective radius), and the blue GCs having β ~ 0.15 at the same radii, while centrally ( ~ 1 Re) they are both isotropic. We discuss the implications of our findings within the two-phase formation scenario for early-type galaxies.
MOND and IMF variations in early-type galaxies from ATLAS3D
C. Tortora, A.J. Romanowsky, V.F. Cardone, N.R. Napolitano, Ph. Jetzer
Monthly Notices Letters of the Royal Astronomical Society, Volume 438, Issue 1, 2014 February 11, pp. L46–L50
Modified Newtonian Dynamics (MOND) represents a phenomenological alternative to dark matter (DM) for the missing mass problem in galaxies and clusters of galaxies. We analyse the central regions of a local sample of ~220 early-type galaxies from the ATLAS3D survey, to see if the data can be reproduced without recourse to DM. We estimate dynamical masses in the MOND context through Jeans analysis and compare to ATLAS3D stellar masses from stellar population synthesis. We find that the observed stellar mass–velocity dispersion relation is steeper than expected assuming MOND with a fixed stellar initial mass function (IMF) and a standard value for the acceleration parameter a0. Turning from the space of observables to model space (a) fixing the IMF, a universal value for a0 cannot be fitted, while, (b) fixing a0 and leaving the IMF free to vary, we find that it is
'lighter' (Chabrier like) for low-dispersion galaxies and 'heavier' (Salpeter like) for high dispersions. This MOND-based trend matches inferences from Newtonian dynamics with DM and from the detailed analysis of spectral absorption lines, adding to the converging lines of evidence for a systematically varying IMF.
The SLUGGS survey: the globular cluster systems of three early-type galaxies using wide-field imaging
S.S. Kartha, D.A. Forbes, L.R. Spitler, A.J. Romanowsky, J.A. Arnold, J.P. Brodie
Monthly Notices of the Royal Astronomical Society, Volume 437, Issue 1, 2014 January 01, pp. 273–292
We present the results from a wide-field imaging study of globular cluster (GC) systems in three early-type galaxies. Combinations of Subaru/Suprime-Cam, Canada-France-Hawaii Telescope/MegaCam and Hubble Space Telescope/Wide Field Planetary Camera 2/Advanced Camera for Surveys data were used to determine the GC system properties of three highly flattened galaxies NGC 720, NGC 1023 and NGC 2768.
This work is the first investigation of the GC system in NGC 720 and NGC 2768 to very large galactocentric radius (~100 kpc). The three galaxies have clear blue and red GC subpopulations. The radial surface densities of the GC systems are fitted with Sérsic profiles, and detected out to 15, 8 and 10 galaxy effective radii, respectively. The total number of GCs and specific frequency are determined for each GC system.
The ellipticity of the red subpopulation is in better agreement with the host galaxy properties than is the blue subpopulation, supporting the traditional view that metal-rich GCs are closely associated with the bulk of their host galaxies' field stars, while metal-poor GCs reflect a distinct stellar halo. With the addition of another 37 literature studied galaxies, we present a new correlation of GC system extent with host galaxy effective radius.
We find a dependence of the relative fraction of blue to red GCs on host galaxy environmental density for lenticular galaxies (but not for elliptical or spiral galaxies). We propose that tidal interactions between galaxies in cluster environments might be the reason behind the observed trend for lenticular galaxies.
The SLUGGS Survey: wide field imaging of the globular cluster system of NGC 4278
C. Usher, D.A. Forbes, L.R. Spitler, J.P. Brodie, A.J. Romanowsky., J. Strader, K.A. Woodley
Monthly Notices of the Royal Astronomical Society, Volume 436, Issue 2, 2013 December 01, pp. 1172–1190
We use multipointing Hubble Space Telescope Advanced Camera for Surveys and wide field
Subaru Suprime-Cam imaging to study the globular cluster system of the L* elliptical galaxy
NGC 4278. We have also obtained a handful of new globular cluster spectra with the Keck
Deep Imaging Multi-Object Spectrograph. We determine the globular cluster surface density
profile and use it to calculate the total number of globular clusters, finding the system to be
slightly more populous than average for galaxies of its luminosity. We find clear evidence for
bimodality in the globular cluster colour distribution and for a colour–magnitude relation in
the blue subpopulation (a 'blue tilt'). We also find negative radial colour gradients in both
colour subpopulations of equal strength which are similar in strength to those reported in
other galaxies. The sizes of NGC 4278's globular clusters decrease with redder colours and
increase with galactocentric radius. The ratio of the sizes of blue to red globular clusters is
independent of galactocentric radius demonstrating that internal effects are responsible for the
size difference with colour.
The SLUGGS survey: outer triaxiality of the fast rotator elliptical NGC 4473
C. Foster, J.A. Arnold, D.A. Forbes, N. Pastorello, A.J. Romanowsky,
L.R. Spitler, J. Strader, J.P. Brodie
Monthly Notices of the Royal Astronomical Society, Volume 435, Issue 4, 2013 November 11, pp. 3587–3591
Systematic surveys of nearby early-type galaxies using integral field unit (IFU) spectrograph data have revealed that galaxies can hide interesting structures only visible through kinematic studies. As part of their pioneering work, the ATLAS3D team have shown that most morphologically elliptical galaxies are centrally kinematically disc like. Hence, while global morphology suggests that ellipticals are ellipsoidal/triaxial in shape, their central kinematics may be consistent with (inclined) oblate systems.
Here, we study the fast rotator elliptical galaxy: NGC 4473. Using slitlets, we obtain galaxy light kinematics out to unprecedentedly large galactocentric radii (2.5 effective radii). While we confirm the IFU results in the central regions, we find that at large galactocentric radii NGC 4473 exhibits a kinematic transition. In the outskirts, we observe clear minor and major axis rotation, a tell-tale sign of triaxiality, which agrees well with the galaxy's Hubble type.
This outer 'kinematically distinct halo' may be expected from simulations of galaxy formation, and in this system contains around one-third of the stellar light. While this galaxy may be a special case, it suggests that further investigation of the outskirts of galaxies is needed to confirm the new paradigm of galaxy classification.
Filling the gap: a new class of old star cluster?
D.A. Forbes, V. Pota, C. Usher, J. Strader, A.J. Romanowsky, J.P. Brodie,
J.A. Arnold, L.R. Spitler
Monthly Notices Letters of the Royal Astronomical Society, Volume 435, Issue 1, 2013 August 1, pp. L6–L10
press release |
It is not understood whether long-lived star clusters possess a continuous range of sizes and masses (and hence densities) or if rather, they should be considered as distinct types with different origins. Utilizing the Hubble Space Telescope to measure sizes and long exposures on the Keck 10-m telescope to obtain distances, we have discovered the first confirmed star clusters that lie within a previously claimed size-luminosity gap dubbed the 'avoidance zone' by Hwang et al. The existence of these star clusters extends the range of sizes, masses and densities for star clusters, and argues against current formation models that predict well-defined size-mass relationships (such as stripped nuclei, giant globular clusters or merged star clusters). The red colour of these gap objects suggests that they are not a new class of object but are related to faint fuzzies observed in nearby lenticular galaxies. We also report a number of low-luminosity ultracompact dwarfs with sizes of up to 50 pc. Future, statistically complete, studies will be encouraged now that it is known that star clusters possess a continuous range of structural properties.
The densest galaxy
J. Strader, A.C. Seth, D.A. Forbes, G. Fabbiano, A.J. Romanowsky, J.P. Brodie, C. Conroy, N. Caldwell, V. Pota, C. Usher, J.A. Arnold
The Astrophysical Journal Letters, Volume 775, Number 1, 2013 September 20, Article L6 (6pp)
press release |
We report the discovery of a remarkable ultra-compact dwarf galaxy around the massive Virgo
elliptical galaxy NGC 4649 (M60), which we call M60-UCD1. With a dynamical mass of 2.0
× 108 M ☉ but a half-light radius of only ~24 pc,
M60-UCD1 is more massive than any ultra-compact dwarfs of comparable size, and is arguably the
densest galaxy known in the local universe. It has a two-component structure well fit by a sum of
Sésic functions, with an elliptical, compact (rh = 14 pc; n ~ 3.3) inner
component and a round, exponential, extended (rh = 49 pc) outer component.
Chandra data reveal a variable central X-ray source with LX ~
1038 erg s–1 that could be an active galactic nucleus associated with a
massive black hole or a low-mass X-ray binary. Analysis of optical spectroscopy shows the object to
be old (>~ 10 Gyr) and of solar metallicity, with elevated
[Mg/Fe] and strongly enhanced [N/Fe] that indicates light-element self-enrichment; such
self-enrichment may be generically present in dense stellar systems. The velocity dispersion (σ ~
70 km s–1) and resulting dynamical mass-to-light ratio (M/LV
= 4.9 ± 0.7) are consistent with – but slightly higher than – expectations for an old,
metal-rich stellar population with a Kroupa initial mass function. The presence of a massive black
hole or a mild increase in low-mass stars or stellar remnants is therefore also consistent with this
M/LV . The stellar density of the galaxy is so high that no dynamical
signature of dark matter is expected. However, the properties of M60-UCD1 suggest an origin in the
tidal stripping of a nucleated galaxy with MB ~ –18 to –19.
The paucity of globular clusters around the field elliptical NGC 7507
J.P. Caso, T. Richtler, L.P. Bassino, R. Salinas, R.R. Lane, A. Romanowsky
Astronomy & Astrophysics, Volume 555, July 2013, Article A56 (8pp)
There is strong evidence that globular cluster systems (GCSs) of massive galaxies are largely assembled by infall/accretion processes. Therefore, we expect the GCSs of isolated elliptical galaxies to be poor. Although not completely isolated, NGC 7507 is a massive field elliptical galaxy with an apparently very low dark matter content.
We determine the richness, the colour distribution, and the structural properties of the GCS of NGC 7507.
We performed wide-field Washington photometry with data obtained with the MOSAIC II camera at the 4m-Blanco telescope (CTIO, Chile).
The GCS is very poor with SN ~= 0.6.
We identify three subpopulations with peaks at (C–T1) colours of 1.21, 1.42, and 1.72. The bluest population may represent the old, metal-poor component. This interpretation is supported by its shallow density profile. The red population is more concentrated, resembling the galaxy light. The intermediate-colour population is strongly peaked in colour, and we interpret this population as the signature of a starburst whose age depends on the metallicity, but should be quite old, since no signatures of a merger are identifiable. In addition, we find a main sequence in the stellar foreground population, which we attribute to the Sagittarius dwarf tidal stream.
The extrordinarily poor GCS of NGC 7507, a massive elliptical galaxy, is an illustration of how important the environmental conditions are for producing rich GCSs.
The SLUGGS survey: probing the supermassive black hole connection with bulges
and haloes using red and blue globular cluster systems
V. Pota, A.W. Graham, D.A. Forbes, A.J. Romanowsky, J.P. Brodie, J. Strader
Monthly Notices of the Royal Astronomical Society, Volume 433, Issue 1, 21 July 2013, pp. 235–242
Understanding whether the bulge or the halo provides the primary link to the growth of supermassive black holes has strong implications for galaxy evolution and supermassive black hole formation itself. In this paper, we approach this issue by investigating extragalactic globular cluster (GC) systems, which can be used to probe the physics of both the bulge and the halo of the host galaxy. We study the relation between the supermassive black hole masses
(MBH) and the GC system velocity dispersions
(σGC) using an updated and improved sample of 21 galaxies. We exploit the dichotomy of GC system colours, to test if the blue and red GCs correlate differently with black hole mass. This may be expected if they trace the potentially different formation history of the halo and of the bulge of the host galaxy, respectively. We find that MBH correlates with the total GC system velocity dispersion, although not as strongly as claimed by recent work of Sadoun & Colin. We also examine the MBH–σGC relation for barred/barless and core/non-core galaxies, finding no significant difference, and for the first time we quantify the impact of radial gradients in the GC system velocity dispersion profile on the MBH–σGC relation. We additionally predict MBH in 13 galaxies, including dwarf elliptical galaxies and the cD galaxy NGC 3311. We conclude that our current results cannot discriminate between the bulge/halo scenarios. Although there is a hint that the red GC velocity dispersion might correlate better with MBH than the blue GC velocity dispersion, the number statistics are still too low to be certain.
Planetary Nebula Spectrograph survey of S0 galaxy kinematics –
II. Clues to the origins of S0 galaxies
A. Cortesi, M.R. Merrifield, L. Coccato, M. Arnaboldi, O. Gerhard, S. Bamford, N.R. Napolitano,
A.J. Romanowsky, N.G. Douglas, K. Kuijken, M. Capaccioli,
K.C. Freeman, K. Saha, A.L. Chies-Santos
Monthly Notices of the Royal Astronomical Society, Volume 432, Issue 2, 21 June 2013, pp. 1010–1020
The stellar kinematics of the spheroids and discs of S0 galaxies contain clues to their formation histories. Unfortunately, it is difficult to disentangle the two components and to recover their stellar kinematics in the faint outer parts of the galaxies using conventional absorption line spectroscopy. This paper therefore presents the stellar kinematics of six S0 galaxies derived from observations of planetary nebulae (PNe), obtained using the Planetary Nebula Spectrograph. To separate the kinematics of the two components, we use a maximum-likelihood method that combines the discrete kinematic data with a photometric component decomposition. The results of this analysis reveal that: the discs of S0 galaxies are rotationally supported; however, the amount of random motion in these discs is systematically higher than in comparable spiral galaxies; and the S0s lie around one magnitude below the Tully–Fisher relation for spiral galaxies, while their spheroids lie nearly one magnitude above the Faber–Jackson relation for ellipticals. All of these findings are consistent with a scenario in which spirals are converted into S0s through a process of mild harassment or "pestering," with their discs somewhat heated and their spheroid somewhat enhanced by the conversion process. In such a scenario, one might expect the properties of S0s to depend on environment. We do not see such an effect in this fairly small sample, although any differences would be diluted by the fact that the current location does not necessarily reflect the environment in which the transformation occurred. Similar observations of larger samples probing a broader range of environments, coupled with more detailed modelling of the transformation process to match the wide range of parameters that we have shown can now be measured, should take us from these first steps to the definitive answer as to how S0 galaxies form.
Angular momentum and galaxy formation revisited: effects of variable
S.M. Fall, A.J. Romanowsky
The Astrophysical Journal Letters, Volume 769, Number 2, 2013 June 1, Article L26 (5pp)
We rederive the relation between the specific angular momentum j*
and the mass M* of the stellar matter in galaxies of different morphological types. This is a revision of the j*–M* diagram presented in our recent comprehensive study of galactic angular momentum. In that work, we estimated j* from kinematic and photometric data that extended to large radii and
M* from near-infrared luminosities LK
with an assumed universal mass-to-light ratio
M*/LK. However, recent stellar population models show large variations in M*/LK
correlated with B–V color. In the present work, we use this correlation to estimate
M*/LK and hence M*
from the measured B–V and LK.
Our revised j*–M* diagram is similar to our previous one; both disk-dominated and elliptical galaxies follow nearly parallel sequences with
alpha = 0.6 ± 0.1. However, the offset between the sequences is now a factor of 5,
some 30% larger than before (and close to the offset found by Fall in 1983). Thus, our new results place even tighter constraints on the loss of specific angular momentum by galactic disks over their lifetimes.
An inventory of the stellar initial mass function in early-type galaxies
C. Tortora, A.J. Romanowsky, N.R. Napolitano
The Astrophysical Journal, Volume 765, Number 1, 2013 March 1, Article 8 (8pp)
Given a flurry of recent claims for systematic variations in the stellar initial mass function (IMF),
we carry out the first inventory of the observational evidence using different approaches. This
includes literature results, as well as our own new findings from combined stellar population synthesis (SPS) and Jeans dynamical analyses of data on ~4500 early-type galaxies (ETGs) from the SPIDER
project. We focus on the mass-to-light ratio mismatch relative to the Milky Way IMF,
deltaIMF, correlated against the central stellar velocity dispersion, σ*.
We find a strong correlation between deltaIMF and σ*,
for a wide set of dark matter (DM) model profiles. These results are robust if a uniform halo response
to baryons is adopted across the sample. The overall normalization of deltaIMF and the
detailed DM profile are less certain, but the data are consistent with standard cold DM halos and a
central DM fraction that is roughly constant with σ*.
For a variety of related studies in the literature, using SPS, dynamics, and gravitational lensing,
similar results are found. Studies based solely on spectroscopic line diagnostics agree on a
Salpeter-like IMF at high σ* but differ at low σ*.
Overall, we find that multiple independent lines of evidence appear to be converging on a systematic
variation in the IMF, such that high-σ* ETGs have an excess of low-mass stars
relative to spirals and low-σ* ETGs. Robust verification of super-Salpeter IMFs
in the highest-σ* galaxies will require additional scrutiny of scatter and
systematic uncertainties. The implications for the distribution of DM are still inconclusive.
The Planetary Nebula Spectrograph survey of S0 galaxy kinematics. Data and overview
A. Cortesi, M. Arnaboldi, L. Coccato, M.R. Merrifield, O. Gerhard, S. Bamford,
A.J. Romanowsky, N.R. Napolitano, N.G. Douglas, K. Kuijken, M. Capaccioli,
K.C. Freeman, A.L. Chies-Santos, V. Pota
Astronomy & Astrophysics, Volume 549, January 2013, Article A115 (12pp)
The origins of S0 galaxies remain obscure, with various mechanisms proposed for their formation, likely depending on environment. These mechanisms would imprint different signatures in the galaxies' stellar kinematics out to large radii, offering a method for distinguishing between them.
We aim to study a sample of six S0 galaxies from a range of environments, and use planetary nebulae (PNe) as tracers of their stellar populations out to very large radii, to determine their kinematics in order to understand their origins.
Using a special-purpose instrument, the Planetary Nebula Spectrograph, we observe and extract PNe catalogues for these six systems.
We show that the PNe have the same spatial distribution as the starlight, that the numbers of them are consistent with what would be expected in a comparable old stellar population in elliptical galaxies, and that their kinematics join smoothly onto those derived at smaller radii from conventional spectroscopy.
The high-quality kinematic observations presented here form an excellent set for studying the detailed kinematics of S0 galaxies, in order to unravel their formation histories. We find that PNe are good tracers of stellar kinematics in these systems. We show that the recovered kinematics are largely dominated by rotational motion, although with significant random velocities in most cases.
The SLUGGS Survey: kinematics for over 2500 globular clusters in twelve early-type galaxies
V. Pota, D.A. Forbes, A.J. Romanowsky, J.P. Brodie, L.R. Spitler, J. Strader, C. Foster, J.A. Arnold, A. Benson, C. Blom, J.R. Hargis, K.L. Rhode, C. Usher
Monthly Notices of the Royal Astronomical Society, Volume 428, Issue 1, 1 January 2013, pp. 389–420
We present a spectrophotometric survey of 2522 extragalactic globular clusters (GCs) around 12 early-type galaxies, nine of which have not been published previously. Combining space-based and multicolour wide-field ground-based imaging, with spectra from the Keck/DEep Imaging Multi-Object Spectrograph (DEIMOS) instrument, we obtain an average of 160 GC radial velocities per galaxy, with a high-velocity precision of ~15 km s–1 per GC. After studying the photometric properties of the GC systems, such as their spatial and colour distributions, we focus on the kinematics of metal-poor (blue) and metal-rich (red) GC subpopulations to an average distance of ~8 effective radii from the galaxy centre.
Our results show that for some systems the bimodality in GC colour is also present in GC kinematics. The kinematics of the red GC subpopulations are strongly coupled with the host galaxy stellar kinematics. The blue GC subpopulations are more dominated by random motions, especially in the outer regions, and decoupled from the red GCs. Peculiar GC kinematic profiles are seen in some galaxies: the blue GCs in NGC 821 rotate along the galaxy minor axis, whereas the GC system of the lenticular galaxy NGC 7457 appears to be strongly rotation supported in the outer region.
We supplement our galaxy sample with data from the literature and carry out a number of tests to study the kinematic differences between the two GC subpopulations. We confirm that the GC kinematics are coupled with the host galaxy properties and find that the velocity kurtosis and the slope of their velocity dispersion profiles are different between the two GC subpopulations in more massive galaxies.
Angular momentum and galaxy formation revisited
A.J. Romanowsky, S.M. Fall
The Astrophysical Journal Supplement Series, Volume 203, Number 2, 2012 December, Article 17 (52pp)
Motivated by a new wave of kinematical tracers
in the outer regions of early-type galaxies (ellipticals and
lenticulars), we re-examine the role of angular momentum in galaxies of
all types. We present new methods for quantifying the specific angular
momentum j, focusing mainly on the more challenging case of
early-type galaxies, in order to derive firm empirical relations between
stellar j* and mass M*
(thus extending earlier work by Fall). We carry out detailed analyses
of eight galaxies with kinematical data extending as far out as 10
effective radii, and find that data at two effective radii are generally
sufficient to estimate total j* reliably. Our results contravene suggestions that ellipticals could harbor large reservoirs of hidden j*
in their outer regions owing to angular momentum transport in major
mergers. We then carry out a comprehensive analysis of extended
kinematic data from the literature for a sample of ~100 nearby bright
galaxies of all types, placing them on a diagram of j* versus M*. The ellipticals and spirals form two parallel j*–M*
tracks, with log-slopes of ~0.6, which for the spirals are closely
related to the Tully–Fisher relation, but for the ellipticals derives
from a remarkable conspiracy between masses, sizes, and rotation
velocities. The ellipticals contain less angular momentum on average
than spirals of equal mass, with the quantitative disparity depending on
the adopted K-band stellar mass-to-light ratios of the galaxies:
it is a factor of ~3–4 if mass-to-light ratio variations are neglected
for simplicity, and ~7 if they are included. We decompose the spirals
into disks and bulges and find that these subcomponents follow j*–M*
trends similar to the overall ones for spirals and ellipticals. The
lenticulars have an intermediate trend, and we propose that the
morphological types of galaxies reflect disk and bulge subcomponents
that follow separate, fundamental j*–M*
scaling relations. This provides a physical motivation for
characterizing galaxies most basically with two parameters: mass and
bulge-to-disk ratio. Next, in an approach complementary to numerical
simulations, we construct idealized models of angular momentum content
in a cosmological context, using estimates of dark matter halo spin and
mass from theoretical and empirical studies. We find that the width of
the halo spin distribution cannot account for the differences between
spiral and elliptical j*, but that the observations are reproduced well if these galaxies simply retained different fractions of their initial j complement (~60% and ~10%, respectively). We consider various physical mechanisms for the simultaneous evolution of j* and M*
(including outflows, stripping, collapse bias, and merging),
emphasizing that the vector sum of all such processes must produce the
relations. We suggest that a combination of early collapse and multiple
mergers (major or minor) may account naturally for the trend for
ellipticals. More generally, the observed variations in angular momentum
represent simple but fundamental constraints for any model of galaxy
The SLUGGS Survey: NGC 3115, a critical test case for metallicity bimodality in globular cluster systems
J.P. Brodie, C. Usher, C. Conroy, J. Strader, J.A. Arnold, D.A. Forbes, A.J. Romanowsky
The Astrophysical Journal Letters, Volume 759, Number 2, 2012 November 10, Article L33 (6pp)
Due to its proximity (9 Mpc) and the strongly bimodal color
distribution of its spectroscopically well-sampled globular cluster
(GC) system, the early-type galaxy NGC 3115 provides one of the best
available tests of whether the color bimodality widely observed in GC
systems generally reflects a true metallicity
bimodality. Color bimodality has alternatively been attributed to a
strongly nonlinear color–metallicity relation reflecting the
influence of hot horizontal-branch stars. Here, we couple Subaru
Suprime-Cam gi photometry with Keck/DEIMOS spectroscopy to
accurately measure GC colors and a CaT index that measures the Ca II
triplet. We find the NGC 3115 GC system to be unambiguously bimodal
in both color and the CaT index. Using simple stellar population
models, we show that the CaT index is essentially unaffected by
variations in horizontal-branch morphology over the range of
metallicities relevant to GC systems (and is thus a robust indicator
of metallicity) and confirm bimodality in the metallicity
distribution. We assess the existing evidence for and against
multiple metallicity subpopulations in early- and late-type galaxies
and conclude that metallicity bi/multimodality is common. We briefly
discuss how this fundamental characteristic links directly to the
star formation and assembly histories of galaxies.
The SLUGGS survey: globular cluster system kinematics and substructure in NGC 4365
C. Blom, D.A. Forbes, J.P. Brodie, C. Foster,
A.J. Romanowsky, L.R. Spitler, J. Strader
Monthly Notices of the Royal Astronomical Society, Volume 426, Issue 3, 1 November 2012, pp. 1959–1971
We present a kinematic analysis of the globular cluster (GC) system of the giant elliptical
galaxy NGC 4365 and find several distinct kinematic substructures. This analysis is carried out
using radial velocities for 269 GCs, obtained with the DEIMOS (DEep Imaging Multi-Object
Spectrograph) instrument on the Keck II telescope as part of the SAGES Legacy Unifying
Globulars and Galaxies Survey (SLUGGS).We find that each of the three (formerly identified)
GC colour subpopulations reveal distinct rotation properties. The rotation of the green GC
subpopulation is consistent with the bulk of NGC 4365's stellar light, which 'rolls' about the
photometric major axis. The blue and red GC subpopulations show `normal' rotation about
the minor axis. We also find that the red GC subpopulation is rotationally dominated beyond
2.5 arcmin (~17 kpc) and that the root mean squared velocity of the green subpopulation
declines sharply with radius suggesting a possible bias towards radial orbits relative to the
other GC subpopulations. Additionally, we find a population of low-velocity GCs that form
a linear structure running from the SW to the NE across NGC 4365 which aligns with the
recently reported stellar stream towards NGC 4342. These low-velocity GCs have
g'–i' colours consistent with the overall NGC 4365 GC system but have velocities consistent with
the systemic velocity of NGC 4342. We discuss the possible formation scenarios for the three
GC subpopulations as well as the possible origin of the low-velocity GC population.
The SLUGGS survey: calcium triplet-based spectroscopic metallicities for over 900 globular clusters
C. Usher, D.A. Forbes, J.P. Brodie, C. Foster, L.R. Spitler, J.A. Arnold, A.J. Romanowsky, J. Strader, V. Pota
Monthly Notices of the Royal Astronomical Society, Volume 426, Issue 2, 21 October 2012, pp. 1475–1495
Although the colour distribution of globular clusters in massive galaxies is well known to be
bimodal, the spectroscopic metallicity distribution has been measured in only a few galaxies.
After redefining the calcium triplet index–metallicity relation, we use our relation to derive
the metallicity of 903 globular clusters in 11 early-type galaxies. This is the largest sample
of spectroscopic globular cluster metallicities yet assembled. We compare these metallicities
with those derived from Lick indices finding good agreement. In six of the eight galaxies with
sufficient numbers of high-quality spectra we find bimodality in the spectroscopic metallicity
distribution. Our results imply that most massive early-type galaxies have bimodal metallicity
as well as colour distributions. This bimodality suggests that most massive early-type galaxies
experienced two periods of star formation.
Radially extended kinematics in the S0 galaxy NGC 2768 from planetary nebulae, globular clusters and starlight
D.A. Forbes, A. Cortesi, V. Pota, C. Foster, A.J. Romanowsky, M.R. Merrifield, J.P. Brodie, J. Strader, L. Coccato, N. Napolitano
Monthly Notices of the Royal Astronomical Society, Volume 426, Issue 2, 21 October 2012, pp. 975–982
There are only a few tracers available to probe the kinematics of individual early-type galaxies
beyond one effective radius. Here we directly compare a sample of planetary nebulae (PNe),
globular clusters (GCs) and galaxy starlight velocities out to approximately four effective
radii, in the S0 galaxy NGC 2768. Using a bulge-to-disc decomposition of a K-band image
we assign PNe and starlight to either the disc or the bulge. We show that the bulge PNe and
bulge starlight follow the same radial density distribution as the red subpopulation of GCs,
whereas the disc PNe and disc starlight are distinct components. We find good kinematic
agreement between the three tracers to several effective radii (and with stellar data in the
inner regions). Further support for the distinct nature of the two galaxy components comes
from our kinematic analysis. After separating the tracers into bulge and disc components we
find the bulge to be a slowly rotating pressure-supported system, whereas the disc reveals a
rapidly rising rotation curve with a declining velocity dispersion profile. The resulting
ratio for the disc resembles that of a spiral galaxy and hints at an origin for NGC 2768 as a
transformed late-type galaxy. A two-component kinematic analysis for a sample of S0s will
help to elucidate the nature of this class of galaxy.
SPIDER – VI. The central dark matter content of luminous early-type galaxies: Benchmark
correlations with mass, structural parameters and environment
C. Tortora, F. La Barbera, N.R. Napolitano, R.R. de Carvalho, A.J. Romanowsky
Monthly Notices of the Royal Astronomical Society, Volume 425, Issue 1, 1 September 2012, pp. 577–594
We analyse the central dark-matter (DM) content of ~4500 massive (M* >~ 1010 M☉), low-redshift (z < 0.1), early-type galaxies (ETGs), with high-quality ugrizY JHK photometry and optical spectroscopy from the Sloan Digital Sky Survey and the UKIRT Infrared Deep Sky Survey–Large Area Survey. We estimate the 'central' fraction of DM within the K-band effective radius, Reff, using spherically symmetric isotropic galaxy models. We discuss the role of systematics in stellar mass estimates, dynamical modelling, and velocity dispersion anisotropy. The main results of the present work are the
following: (1) DM fractions increase systematically with both structural parameters (i.e. Reff
and Sésic index, n) and mass proxies (central velocity dispersion, stellar and dynamical
mass), as in previous studies, and decrease with central stellar density. (2) All correlations involving DM fractions are caused by two fundamental ones with galaxy effective radius and central velocity dispersion. These correlations are independent of each other, so that ETGs populate a central-DM plane (DMP), i.e. a correlation among fraction of total-to-stellar mass, effective radius, and velocity dispersion, whose scatter along the total-to-stellar mass axis
amounts to ~0.15 dex. (3) In general, under the assumption of an isothermal or a constant
M/L profile for the total mass distribution, a Chabrier initial mass function (IMF) is favoured
with respect to a bottom-heavier Salpeter IMF, as the latter produces negative (i.e. unphysical) DM fractions for more than 50 per cent of the galaxies in our sample. For a Chabrier IMF, the DM estimates agree with Λ cold dark matter toy-galaxy models based on contracted DM-halo density profiles. We also find agreement with predictions from hydrodynamical simulations. (4) The central DM content of ETGs does not depend significantly on the environment where galaxies reside, with group and field ETGs having similar DM trends.
The globular cluster system of NGC 1316. II. The extraordinary object SH2
T. Richtler, B. Kumar, L.P. Bassino, B. Dirsch, A.J. Romanowsky
Astronomy & Astrophysics, Volume 543, July 2012, Article L7 (4pp)
SH2 has been described as an isolated HII-region, located about 6.5 south of the nucleus of NGC 1316 (Fornax A), a merger remnant in the the outskirts of the Fornax cluster of galaxies.
We give a first, preliminary description of the stellar content and environment of this remarkable object.
We used photometric data in the Washington system and HST photometry from the Hubble Legacy Archive for a morphological description and preliminary aperture photometry. Low-resolution spectroscopy provides radial velocities of the brightest star cluster in SH2 and a nearby intermediate-age cluster.
SH2 is not a normal HII-region, ionized by very young stars. It contains a multitude of star clusters with ages of approximately 108 yr. A ring-like morphology is striking. SH2 seems to be connected to an intermediate-age massive globular cluster with a similar radial velocity, which itself is the main object of a group of fainter clusters. Metallicity estimates from emission lines remain ambiguous.
The present data do not yet allow firm conclusions about the nature or origin of SH2. It might be a dwarf galaxy that has experienced a burst of extremely clustered star formation. We may witness how globular clusters are donated to a parent galaxy.
Evidence for inhomogeneous reionization in the local Universe from metal-poor globular cluster systems
L.R. Spitler, A.J. Romanowsky, J. Diemand, J. Strader, D.A. Forbes, B. Moore, J.P. Brodie
Monthly Notices of the Royal Astronomical Society, Volume 423, Issue 3, July 2012, pp. 2177–2189
Exploiting a fundamental characteristic of galaxy assembly in the Λ cold dark matter paradigm,
the observed spatial biasing and kinematics of metal-poor globular star clusters are used to
constrain the local reionization epoch around individual galaxies. Selecting three galaxies
located in different environments, the first attempt at constraining the environmental propagation
of reionization in the local Universe is carried out. The joint constraint from the three galaxies
(zreion= 10.5+1.0-0.9) agrees remarkably well with the latest Wilkinson Microwave Anisotropy
Probe constraint on zreion for a simple instantaneous reionization model.
More importantly, the range of zreion values found here is consistent with the global range of zreion estimates from other observations. We furthermore find a 1.7 σ indication that reionization completed in low-density environments before the intergalactic medium in high-density environments was reionized. This is consistent with certain theoretical models that predict that reionization was globally prolonged in duration, with neutral hydrogen pockets surviving in high-density environments, even after the surrounding regions were reionized. More generally, this work provides a useful constraint on the formation history of galaxy stellar haloes.
Dwarfs gobbling dwarfs: A stellar tidal stream around NGC 4449 and hierarchical galaxy formation on small scales
D. Martínez-Delgado, A.J. Romanowsky, R.J. GaBany, F. Annibali, J.A. Arnold, J. Fliri, S. Zibetti, R.P. van der Marel, H.-W. Rix, T.S. Chonis, J.A. Carballo-Bello, A. Aloisi, A.V. Maccio, J. Gallego-Laborda, J.P. Brodie, M.R. Merrifield
The Astrophysical Journal Letters, Volume 748, Number 2, 2012 April 1, Article L24 (6pp)
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A candidate diffuse stellar substructure was previously reported in the halo of the nearby dwarf starburst galaxy NGC 4449 by Karachentsev et al. We map and analyze this feature using a unique combination of deep integrated-light images from the BlackBird 0.5 m telescope, and high-resolution wide-field images from the 8 m Subaru Telescope, which resolve the nebulosity into a stream of red giant branch stars, and confirm its physical association with NGC 4449. The properties of the stream imply a massive dwarf spheroidal progenitor, which after complete disruption will deposit an amount of stellar mass that is comparable to the existing stellar halo of the main galaxy. The stellar mass ratio between the two galaxies is ~1:50, while the indirectly measured dynamical mass ratio, when including dark matter, may be ~1:10–1:5. This system may thus represent a "stealth" merger, where an infalling satellite galaxy is nearly undetectable by conventional means, yet has a substantial dynamical influence on its host galaxy. This singular discovery also suggests that satellite accretion can play a significant role in building up the stellar halos of low-mass galaxies, and possibly in triggering their starbursts.
Testing Yukawa-like potentials from f(R)-gravity in elliptical galaxies
N.R. Napolitano, S. Capozziello, A.J. Romanowsky, M. Capaccioli, C. Tortora
The Astrophysical Journal, Volume 748, Number 2, 2012 April 1, Article 87 (6pp)
We present the first analysis of extended stellar kinematics of elliptical galaxies where a Yukawa-like correction
to the Newtonian gravitational potential derived from f(R)-gravity is considered as an alternative to dark matter.
In this framework, we model long-slit data and planetary nebula data out to 7 Reff of three galaxies with either
decreasing or flat dispersion profiles. We use the corrected Newtonian potential in a dispersionkurtosis Jeans
analysis to account for the massanisotropy degeneracy. We find that these modified potentials are able to fit
nicely all three elliptical galaxies and the anisotropy distribution is consistent with that estimated if a dark halo
is considered. The parameter which measures the strength of the Yukawa-like correction is, on average, smaller
than the one found previously in spiral galaxies and correlates both with the scale length of the Yukawa-like term
and the orbital anisotropy.
The ongoing assembly of a central cluster galaxy: phase-space substructures in the halo of M87
A.J. Romanowsky, J. Strader, J.P. Brodie, J.C. Mihos, L.R. Spitler, D.A. Forbes, C. Foster, J.A. Arnold
The Astrophysical Journal, Volume 748, Number 1, 2012 March 20, Article 29 (23pp)
The halos of galaxies preserve unique records of their formation histories. We carry out the first combined observational and theoretical study of phase-space halo substructure in an early-type galaxy: M87, the central galaxy in the Virgo cluster. We analyze an unprecedented wide-field, high-precision photometric and spectroscopic data set for 488 globular clusters (GCs), which includes new, large-radius Subaru/Suprime-Cam and Keck/DEIMOS observations. We find signatures of two substructures in position-velocity phase space. One is a small, cold stream associated with a known stellar filament in the outer halo; the other is a large shell-like pattern in the inner halo that implies a massive, hitherto unrecognized accretion event. We perform extensive statistical tests and independent metallicity analyses to verify the presence and characterize the properties of these features, and to provide more general methodologies for future extragalactic studies of phase-space substructure. The cold outer stream is consistent with a dwarf galaxy accretion event, while for the inner shell there is tension between a low progenitor mass implied by the cold velocity dispersion, and a high mass from the large number of GCs, which might be resolved by a ~0.5 L* E/S0 progenitor. We also carry out proof-of-principle numerical simulations of the accretion of smaller galaxies in an M87-like gravitational potential. These produce analogous features to the observed substructures, which should have observable lifetimes of ~1 Gyr. The shell and stream GCs together support a scenario where the extended stellar envelope of M87 has been built up by a steady rain of material that continues until the present day. This phase-space method demonstrates unique potential for detailed tests of galaxy formation beyond the Local Group.
Kinematic properties of the field elliptical NGC 7507
R. Salinas, T. Richtler, L.P. Bassino, A.J. Romanowsky, Y. Schuberth
Astronomy & Astrophysics, Volume 538, February 2012, Article A87 (17pp)
The dark matter (DM) halos of field elliptical galaxies have not been well-studied and their properties appear controversial in the literature. While some galaxies appear to be nearly devoid of DM, others show clear evidence of its presence. Furthermore, modified Newtonian
dynamics (MOND), which has been found to have predictive power in the domain of disk galaxies, has not yet been investigated for isolated elliptical galaxies. We study the kinematics of the isolated elliptical NGC 7507, which has been claimed as a clear example of DM presence in early-type galaxies. We obtained major and minor axis long-slit spectroscopy of NGC 7507 using the Gemini South telescope and deep imaging in Kron-Cousins R and Washington C using the CTIO/MOSAIC camera. Mean velocities, velocity dispersion and higher order
moments of the velocity distribution are measured out to ~90". The galaxy, although almost circular, has significant rotation along the minor axis and a rapidly declining velocity dispersion along both axes. The velocity dispersion profile is modeled in the context of a
spherical Jeans analysis. Models without DM provide an excellent representation of the data with a mass-to-light ratio (M/L) of 3.1 (R-band). The most massive
Navarro-Frenk-White (NFW) halo the data allow has a virial mass of only 3.9-2.1+3.1×1011M☉, although the data are more consistent with models that have a slight radial anisotropy, which implies the galaxy has an even lower
DM halo mass of 2.2–1.2+2.0×1011M☉. Modeling of the h4 Gauss-Hermite coefficient is inconclusive but seems to be consistent with mild radial anisotropy. A cored logarithmic DM halo with
parameters r0=7 kpc and v0=100 km s–1 can also reproduce the observed velocity dispersion profile. The MOND predictions overestimate
the velocity dispersion. In conclusion, we cannot easily reproduce the previous findings of a predominance of DM in NGC 7507 within a simple spherical model. DM may be present, but only in conjunction with a strong radial anisotropy, for which there are some indications.
Stellar mass-to-light ratio gradients in galaxies: correlations with mass
C. Tortora, N.R. Napolitano, A.J. Romanowsky, Ph. Jetzer, V.F. Cardone, M. Capaccioli
Monthly Notices of the Royal Astronomical Society, Volume 418, Issue 3, 2011 December 11, pp. 1557–1564
We analyze the stellar mass-to-light ratio (M/L) gradients in a large sample of
local galaxies taken from the Sloan Digital Sky Survey, spanning a wide range of stellar
masses and morphological types. As suggested by the well known relationship between
mass-to-light (M/L) ratios and colors, we show that M/L gradients are strongly corre-
lated with colour gradients, which we trace to the effects of age variations. Stellar M/L
gradients generally follow patterns of variation with stellar mass and galaxy type that
were previous found for colour and metallicity gradients. In late-type galaxies M/L
gradients are negative, steepening with increasing mass. In early-type galaxies M/L
gradients are shallower while presenting a two-fold trend: they decrease with mass up
to a characteristic mass of M*~1010.3M☉
and increase at larger masses. We compare
our findings with other analyses and discuss some implications for galaxy formation
and for dark matter estimates.
Wide-field precision kinematics of the M87 globular cluster system
J. Strader, A.J. Romanowsky, J.P. Brodie, L.R. Spitler, M.A. Beasley, J.A. Arnold, N. Tamura, R.M. Sharples, N. Arimoto
The Astrophysical Journal Supplement Series, Volume 197, Number 2, 2011 December, Article 33 (49pp)
We present the most extensive combined photometric and spectroscopic study to date of the enormous globular cluster (GC) system around M87, the central giant elliptical galaxy in the nearby Virgo Cluster. Using observations from DEIMOS and the Low Resolution Imaging Spectrometer at Keck, and Hectospec on the Multiple Mirror Telescope, we derive new, precise radial velocities for 451 GCs around M87, with projected radii from ~5 to 185 kpc. We combine these measurements with literature data for a total sample of 737 objects, which we use for a re-examination of the kinematics of the GC system of M87. The velocities are analyzed in the context of archival wide-field photometry and a novel Hubble Space Telescope catalog of half-light radii, which includes sizes for 344 spectroscopically confirmed clusters. We use this unique catalog to identify 18 new candidate ultracompact dwarfs and to help clarify the relationship between these objects and true GCs. We find much lower values for the outer velocity dispersion and rotation of the GC system than in earlier papers and also differ from previous work in seeing no evidence for a transition in the inner halo to a potential dominated by the Virgo Cluster, nor for a truncation of the stellar halo. We find little kinematical evidence for an intergalactic GC population. Aided by the precision of the new velocity measurements, we see significant evidence for kinematical substructure over a wide range of radii, indicating that M87 is in active assembly. A simple, scale-free analysis finds less dark matter within ~85 kpc than in other recent work, reducing the tension between X-ray and optical results. In general, out to a projected radius of ~150 kpc, our data are consistent with the notion that M87 is not dynamically coupled to the Virgo Cluster; the core of Virgo may be in the earliest stages of assembly.
The relationships among compact stellar systems: a fresh view of ultra compact dwarfs
J.P. Brodie, A.J. Romanowsky, J. Strader, D.A. Forbes
The Astronomical Journal, Volume 142, Number 6, 2011 December, Article 199 (16pp)
We use a combined imaging and spectroscopic survey of the nearby central cluster galaxy, M87, to assemble
a sample of 34 confirmed ultracompact dwarfs (UCDs) with half-light radii of >~ 10 pc measured from Hubble
Space Telescope images. This doubles the existing sample in M87, making it the largest such sample for any
galaxy, while extending the detection of UCDs to unprecedentedly low luminosities (MV = -9). With this
expanded sample, we find no correlation between size and luminosity, in contrast to previous suggestions, and
no general correlation between size and galactocentric distance. We explore the relationships between UCDs, less
luminous extended clusters (including faint fuzzies), globular clusters (GCs), as well as early-type galaxies and their
nuclei, assembling an extensive new catalog of sizes and luminosities for stellar systems. Most of the M87 UCDs
follow a tight color-magnitude relation, offset from the metal-poor GCs. This, along with kinematical differences,
demonstrates that most UCDs are a distinct population from normal GCs, and not simply a continuation to larger
sizes and higher luminosities. The UCD color-magnitude trend couples closely with that for Virgo dwarf elliptical
nuclei. We conclude that the M87 UCDs are predominantly stripped nuclei. The brightest and reddest UCDs may be
the remnant nuclei of more massive galaxies while a subset of the faintest UCDs may be tidally limited and related
to more compact star clusters. In the broader context of galaxy assembly, blue UCDs may trace halo build-up by
accretion of low-mass satellites, while red UCDs may be markers of metal-rich bulge formation in larger galaxies.
Galaxies in ΛCDM with Halo Abundance Matching: luminosity-velocity relation, baryonic mass-velocity relation, velocity function, and clustering
S. Trujillo-Gomez, A. Klypin, J. Primack, A.J. Romanowsky
The Astrophysical Journal, Volume 742, Number 1, 2011 November 20, Article 16 (23pp)
It has long been regarded as difficult if not impossible for a cosmological model to account
simultaneously for the galaxy luminosity, mass, and velocity distributions. We revisit this issue using a modern compilation of observational data along with the best available large-scale
cosmological simulation of dark matter (DM). We find that the standard cosmological model,
used in conjunction with halo abundance matching (HAM) and simple dynamical corrections,
fits – at least on average – all basic statistics of galaxies with circular velocities
Vcirc > 80 km s–1 calculated at a radius of ~10 kpc. Our
primary observational constraint is the luminosity-velocity (LV) relation – which
generalizes the Tully-Fisher and Faber-Jackson relations in allowing all types of galaxies to be
included, and provides a fundamental benchmark to be reproduced by any theory of galaxy formation.
We have compiled data for a variety of galaxies ranging from dwarf irregulars to giant
ellipticals. The data present a clear monotonic LV relation from ~50 km s-– to
~500 km s–1, with a bend below ~80 km s–1 and a systematic offset between late-
and early-type galaxies. For comparison to theory, we employ our new ΛCDM "Bolshoi"
simulation of DM, which has unprecedented mass and force resolution over a large cosmological
volume, while using an up-to-date set of cosmological parameters. We use HAM to assign
rank-ordered galaxy luminosities to the DM halos, a procedure that automatically fits the
empirical luminosity function and provides a predicted LV relation that can be checked against
observations. The adiabatic contraction of DM halos in response to the infall of the baryons
is included as an optional model ingredient. The resulting predictions for the LV relation
are in excellent agreement with the available data on both early-type and late-type galaxies
for the luminosity range from Mr = –14 to Mr = -22.
We also compare our predictions for the "cold" baryon mass (i.e., stars and cold gas) of
galaxies as a function of circular velocity with the available observations, again finding a
very good agreement. The predicted circular velocity function (VF) is also in agreement with
the galaxy VF from 80 to 400 km s–1, using the HIPASS survey for late-type galaxies
and Sloan Digital Sky Survey (SDSS) for early-type galaxies. However, in accord with other
recent results, we find that the DM halos with Vcirc < 80 km s–1
are much more abundant than observed galaxies with the same Vcirc. Finally,
we find that the two-point correlation function of bright galaxies in our model matches
very well the results from the final data release of the SDSS, especially when a small
amount of scatter is included in the HAM prescription.
Global properties of 'ordinary' early-type galaxies: photometry and spectroscopy
of stars and globular clusters in NGC 4494
C. Foster, L.R. Spitler, A.J. Romanowsky, D.A. Forbes, V. Pota, K. Bekki, J. Strader, R.N. Proctor, J.A. Arnold, J.P. Brodie
Monthly Notices of the Royal Astronomical Society, Volume 415, Issue 4, 2011 August 21, pp. 3393–3416 (erratum: Volume 456, Issue 2, 2016 Feburary 21, p. 2172)
We present a comprehensive analysis of the spatial, kinematic, and chemical
properties of stars and globular clusters (GCs) in the 'ordinary' elliptical galaxy NGC 4494
using data from the Keck and Subaru telescopes. We derive galaxy surface
brightness and colour profiles out to large galactocentric radii. We compare the latter to
metallicities derived using the near-infrared Calcium Triplet. We obtain stellar
kinematics out to ~3.5 effective radii. The latter appear flattened or elongated beyond
~1.8 effective radii in contrast to the relatively round photometric isophotes. In fact,
NGC 4494 may be a flattened galaxy, possibly even an S0, seen at an inclination of
~45 degrees. We publish a catalogue of 431 GC candidates brighter than i0 = 24
based on the photometry, of which 109 are confirmed spectroscopically and 54 have
measured spectroscopic metallicities. We also report the discovery of 3
spectroscopically confirmed ultra-compact dwarfs around NGC 4494 with measured metallicities
of -0.4 <~ [Fe/H] <~ -0.3. Based on their properties, we conclude that they are simply
bright GCs. The metal-poor globular clusters are found to be rotating with similar
amplitude as the galaxy stars, while the metal-rich globular clusters show marginal
rotation. We supplement our analysis with available literature data and results. Using
model predictions of galaxy formation, and a suite of merger simulations, we find that
many of the observational properties of NGC 4494 may be explained by formation
in a relatively recent gas-rich major merger. Complete studies of individual galaxies
incorporating a range of observational avenues and methods such as the one presented
here will be an invaluable tool for constraining the fine details of galaxy formation
models, especially at large galactocentric radii.
The fossil record of two-phase galaxy assembly: Kinematics and metallicities in the nearest S0 galaxy
J.A. Arnold, A.J. Romanowsky, J.P. Brodie, L. Chomiuk, L.R. Spitler, J. Strader, A.J. Benson, D.A. Forbes
The Astrophysical Journal Letters, Volume 736, Number 2, 2011 August 1, Article L26 (5pp)
We present a global analysis of kinematics and metallicity in the nearest S0 galaxy, NGC 3115, along with implications for its assembly history. The data include high-quality wide-field imaging from Suprime-Cam on the Subaru telescope, and multi-slit spectra of the field stars and globular clusters (GCs) obtained using Keck-DEIMOS/LRIS and Magellan-IMACS. Within two effective radii, the bulge (as traced by the stars and metal-rich GCs) is flattened and rotates rapidly (v/σ >~ 1.5). At larger radii, the rotation declines dramatically to v/σ ~ 0.7, but remains well aligned with the inner regions. The radial decrease in characteristic metallicity of both the metal-rich and metal-poor GC subpopulations produces strong gradients with power-law slopes of -0.17 ± 0.04 and -0.38 ± 0.06 dex dex–1, respectively. We argue that this pattern is not naturally explained by a binary major merger, but instead by a two-phase assembly process where the inner regions have formed in an early violent, dissipative phase, followed by the protracted growth of the outer parts via minor mergers with typical mass ratios of ~15–20:1.
An extremely luminous panchromatic outburst from the nucleus of a distant galaxy
A.J. Levan, N.R. Tanvir, S.B. Cenko, D.A. Perley, K. Wiersema, J.S. Bloom, A.S. Fruchter, A. de Ugarte Postigo, P.T. O'Brien, N. Butler, A.J. van der Horst, G. Leloudas, A.N. Morgan, K. Misra, G.C. Bower, J. Farihi, R.L. Tunnicliffe, M. Modjaz, J.M. Silverman, J. Hjorth, C. Thöne, A. Cucchiara, J.M. Castro Cerón, A.J. Castro-Tirado, J.A. Arnold, M. Bremer, J.P. Brodie, T. Carroll, M.C. Cooper, P.A. Curran, R.M. Cutri, J. Ehle, D. Forbes, J. Fynbo, J. Gorosabel, J. Graham, D.I. Hoffman, S. Guziy, P. Jakobsson, A. Kamble, T. Kerr, M.M. Kasliwal, C. Kouveliotou, D. Kocevski, N.M. Law, P.E. Nugent, E.O. Ofek, D. Poznanski, R.M. Quimby, E. Rol, A.J. Romanowsky, R. Sánchez-Ramírez, S. Schulze, N. Singh, L. van Spaandonk, R.L.C. Starling, R.G. Strom, J.C. Tello, O. Vaduvescu, P.J. Wheatley, R.A.M.J. Wijers, J.M. Winters, D. Xu
Science, Volume 333, Number 6039, 8 July 2011, pp. 199–202
Science Express |
Variable x-ray and gamma-ray emission is characteristic of the most extreme physical processes in the universe. We present multiwavelength observations of a unique gamma-ray-selected transient detected by the Swift satellite, accompanied by bright emission across the electromagnetic spectrum, and whose properties are unlike any previously observed source. We pinpoint the event to the center of a small, star-forming galaxy at redshift z = 0.3534. Its high-energy emission has lasted much longer than any gamma-ray burst, whereas its peak luminosity was 100 times higher than bright active galactic nuclei. The association of the outburst with the center of its host galaxy suggests that this phenomenon has its origin in a rare mechanism involving the massive black hole in the nucleus of that galaxy.
The dark halo of the Hydra I galaxy cluster: core, cusp, cosmological? Dynamics of NGC 3311 and its globular cluster system
T. Richtler, R. Salinas, I. Misgeld, M. Hilker, G.K.T. Hau, A.J. Romanowsky, Y. Schuberth, M. Spolaor
Astronomy & Astrophysics, Volume 531, July 2011, Article A119 (8pp)
Some galaxy clusters exhibit shallow or even cored dark matter density profiles in their central regions rather than the
predicted steep or cuspy profiles, conflicting with the standard understanding of dark matter. NGC 3311 is the central cD galaxy of
the Hydra I cluster (Abell 1060).
We use globular clusters around NGC 3311, combined with kinematical data of the galaxy itself, to investigate the dark matter
distribution in the central region of Hydra I.
Radial velocities of 118 bright globular clusters, based on VLT/VIMOS mask spectroscopy, are used to calculate velocity
dispersions which are well defined out to 100 kpc. NGC 3311 is the most distant galaxy for which this kind of study has been
performed. We also determine the velocity dispersions of the stellar component from long-slit spectroscopy of NGC 3311 acquired
with VLT/FORS1 out to 20 kpc. We present a new photometric model for NGC 3311, based on deep VLT/FORS1 images in the
V-band. We search for a dark halo that, in the context of a spherical Jeans model, can reproduce the kinematical data. We also
compare the radial velocity distributions of globular clusters and planetary nebulae.
The projected stellar velocity dispersion rises from a central low value of about 185 km s–1 to 350 km s–1 at a radius of 20 kpc.
The globular cluster dispersion rises as well from 500 km s–1 at 10 kpc to about 800 km s–1 at 100 kpc, comparable to the velocity
dispersion of the cluster galaxies. A dark matter halo with a core (Burkert halo) closely reproduces the velocity dispersions of stars
and globular clusters simultaneously under isotropy. The central stellar velocity dispersions predicted by cosmological NFW halos do
not agree well with those observed, while the globular clusters allow a wide range of halo parameters. A suspected radial anisotropy
of the stellar population found in merger simulations aggravates the disagreement with observations. A slight tangential anisotropy
would enable the data to be more accurately reproduced. However, we find discrepancies with previous kinematical data that we
cannot resolve, which may indicate a more complicated velocity pattern.
Although one cannot conclusively demonstrate that the dark matter halo of NGC 3311 has a core rather than a cusp, a
core seems to be most consistent with the present data. A more complete coverage of the velocity field and a more thorough analysis
of the anisotropy is required to reach firm conclusions.
Unravelling the origins of S0 galaxies using maximum likelihood analysis of planetary nebulae kinematics
A. Cortesi, M.R. Merrifield, M. Arnaboldi, O. Gerhard, I. Martinez-Valpuesta, K. Saha,
L. Coccato, S. Bamford, N.R. Napolitano, P. Das, N.G. Douglas,
A.J. Romanowsky, K. Kuijken, M. Capaccioli, K.C. Freeman
Monthly Notices of the Royal Astronomical Society, Volume 414, Issue 1, June 2011, pp. 642–651
To investigate the origins of S0 galaxies, we present a new method of analysing their stellar kinematics from discrete tracers such as planetary nebulae. This method involves binning the data in the radial direction so as to extract the most general possible non-parametric kinematic profiles, and using a maximum-likelihood fit within each bin in order to make full use of the information in the discrete kinematic tracers. Both disc and spheroid kinematic components are fitted, with a two-dimensional decomposition of imaging data used to attribute to each tracer a probability of membership in the separate components. Likelihood clipping also allows us to identify objects whose properties are not consistent with the adopted model, rendering the technique robust against contaminants and able to identify additional kinematic features.
The method is first tested on an N-body simulated galaxy to assess possible sources of systematic error associated with the structural and kinematic decomposition, which are found to be small. It is then applied to the S0 system NGC 1023, for which a planetary nebula catalogue has already been released and analysed by Noordermer et al. The correct inclusion of the spheroidal component allows us to show that, contrary to previous claims, the stellar kinematics of this galaxy are indistinguishable from those of a normal spiral galaxy, indicating that it may have evolved directly from such a system via gas stripping or secular evolution. The method also successfully identifies a population of outliers whose kinematics are different from those of the main galaxy; these objects can be identified with a stellar stream associated with the companion galaxy NGC 1023A.
Evidence for two phases of galaxy formation from radial trends in the globular cluster system of NGC 1407
D.A. Forbes, L.R. Spitler, J. Strader, A.J. Romanowsky, J.P. Brodie, C. Foster
Monthly Notices of the Royal Astronomical Society, Volume 413, Issue 4, June 2011, pp. 2943–2949
Here we present the colours of individual globular clusters (GCs) around the massive elliptical galaxy NGC 1407 out to a projected galactocentric radius of 140 kpc or 17 galaxy effective radii (Re). Such data are a proxy for the halo metallicity. We find steep, and similar, metallicity gradients of ~0.4 dex dex–1 for both the blue (metal poor) and red (metal rich) GC subpopulations within 5–8.5 Re (40–70 kpc). At larger radii the mean GC colours (metallicity) are constant. A similar behaviour is seen in a wide-field study of M87's GC system, and in our own Galaxy. We interpret these radial metallicity trends to indicate an inner region formed by early in situ dissipative processes and an outer halo formed by the ongoing accretion of low-mass galaxies and their GCs. These results provide observational support for the model of galaxy formation whereby massive galaxies form inside-out in two phases. We have also searched the literature for other massive early-type galaxies with reported GC metallicity gradients in their inner regions. No obvious correlation with galaxy mass or environment is found but the sample is currently small.
Optical and near-infrared velocity dispersions of early-type galaxies
J. Vanderbeke, M. Baes, A.J. Romanowsky, L. Schmidtobreick
Monthly Notices of the Royal Astronomical Society, Volume 412, Issue 3, April 2011, pp. 2017–2025
We have carried out a systematic, homogeneous comparison of optical and near-infrared dispersions. Our magnitude-limited sample of early-type galaxies (ETGs) in the Fornax cluster comprises 11 elliptical and 11 lenticular galaxies more luminous than MB=–17. We were able to determine the central dispersions based on the near-infrared CO absorption band head for 19 of those galaxies. The velocity dispersions range from less than 70 km s–1 to over 400 km s–1. We compare our near-infrared velocity dispersions to the optical dispersions measured by Kuntschner. Contrary to previous studies, we find a one-to-one correspondence with a median fractional difference of 6.4 per cent. We examine the correlation between the relative dust mass and the fractional difference of the velocity dispersions, but find no significant trend. Our results suggest that early-type galaxies are largely optically thin, which is consistent with recent Herschel observations.
Crazy heart: kinematics of the "star pile" in Abell 545
R. Salinas, T. Richtler, M.J. West, A.J. Romanowsky, E. Lloyd-Davies, Y. Schuberth
Astronomy & Astrophysics, Volume 528, April 2011, Article A61 (9pp)
We study the structure and internal kinematics of the "star pile" in Abell 545 – a low surface brightness structure lying in the center
of the cluster.We have obtained deep long-slit spectroscopy of the star pile using VLT/FORS2 and Gemini/GMOS, which is analyzed
in conjunction with deep multiband CFHT/MEGACAM imaging. As presented in a previous study the star pile has a flat luminosity
profile and its color is consistent with the outer parts of elliptical galaxies. Its velocity map is irregular, with parts being seemingly
associated with an embedded nucleus, and others which have significant velocity offsets to the cluster systemic velocity with no clear
kinematical connection to any of the surrounding galaxies. This would make the star pile a dynamically defined stellar intra-cluster
component. The complicated pattern in velocity and velocity dispersions casts doubts on the adequacy of using the whole star pile
as a dynamical test for the innermost dark matter profile of the cluster. This status is fulfilled only by the nucleus and its nearest
surroundings which lie at the center of the cluster velocity distribution.
The PN.S Elliptical Galaxy Survey: a standard ΛCDM halo around NGC 4374?
N.R. Napolitano, A.J. Romanowsky, M. Capaccioli, N.G. Douglas, M. Arnaboldi,
L. Coccato, O. Gerhard, K. Kuijken, M.R. Merrifield, S.P. Bamford, A. Cortesi, P. Das,
Monthly Notices of the Royal Astronomical Society, Volume 411, Issue 3, March 2011, pp. 2035–2053
As part of our current programme to test ΛCDM predictions for dark matter (DM) haloes using extended kinematical observations of early-type galaxies, we present a dynamical analysis of the bright elliptical galaxy NGC 4374 (M84) based on ~450 Planetary Nebulae (PNe) velocities from the PN.Spectrograph, along with extended long-slit stellar kinematics. This is the first such analysis of a galaxy from our survey with a radially constant velocity dispersion profile. We find that the spatial and kinematical distributions of the PNe agree with the field stars in the region of overlap. The velocity kurtosis is consistent with zero at almost all radii. We construct a series of Jeans models, fitting both velocity dispersion and kurtosis to help break the mass-anisotropy degeneracy. Our mass models include DM halos either with shallow cores or with central cusps as predicted by cosmological simulations - along with the novel introduction in this context of adiabatic halo contraction from baryon infall. Both classes of models confirm a very massive dark halo around NGC 4374, demonstrating that PN kinematics data are well able to detect such haloes when present. Considering the default cosmological mass model, we confirm earlier suggestions that bright galaxies tend to have halo concentrations higher than ΛCDM predictions, but this is found to be solved if either a Salpeter IMF or adiabatic contraction with a Kroupa IMF is assumed. Thus for the first time a case is found where the PN dynamics may well be consistent with a standard dark matter halo. A cored halo can also fit the data, and prefers a stellar mass consistent with a Salpeter IMF. The less dramatic dark matter content found in lower-luminosity "ordinary" ellipticals suggests a bimodality in the halo properties which may be produced by divergent baryonic effects during their assembly histories.
Star clusters in M31: Old clusters with bar kinematics
H. Morrison, N. Caldwell, R.P. Schiavon, E. Athanassoula, A.J. Romanowsky, P. Harding
The Astrophysical Journal Letters, Volume 726, Number 1, 2011 January 1, Article L9 (4pp)
We analyze our accurate kinematical data for the old clusters in the inner regions of M31. These velocities are
based on high signal-to-noise Hectospec data. The data are well suited for analysis of M31's inner regions because
we took particular care to correct for contamination by unresolved field stars from the disk and bulge in the fibers.
The metal-poor clusters show kinematics that are compatible with a pressure-supported spheroid. The kinematics
of metal-rich clusters, however, argue for a disk population. In particular the innermost region (inside 2 kpc) shows
the kinematics of the x2 family of bar periodic orbits, arguing for the existence of an inner Lindblad resonance in
Central dark matter trends in early-type galaxies from strong
lensing, dynamics and stellar populations
C. Tortora, N.R. Napolitano, A.J. Romanowsky, P. Jetzer
The Astrophysical Journal Letters, Volume 721, Number 1, 2010 September 20, L1–L5
We analyze the correlations between central dark matter (DM) content of
early-type galaxies and their sizes and ages, using a sample of
intermediate-redshift (z ~ 0.2) gravitational lenses from the SLACS
by comparing them to a larger sample of z ~ 0 galaxies. We decompose the
deprojected galaxy masses into DM and stellar components using
strong lensing, stellar dynamics, and stellar populations modeling. For a
stellar mass, we find that for galaxies with larger sizes, the DM fraction
increases and the mean DM density decreases, consistently with the cuspy
expected in cosmological formation scenarios. The DM fraction also decreases
with stellar age, which can be partially explained by the inverse correlation
between size and age. The residual trend may point to systematic dependencies
on formation epoch of halo contraction or stellar initial mass functions.
results are in agreement with recent findings based on local galaxies by
Napolitano et al. and suggest negligible evidence of
galaxy evolution over the last ~ 2.5 Gyr other than passive stellar aging.
The central dark matter content of early-type galaxies: scaling relations and connections with
star formation histories
N.R. Napolitano, A.J. Romanowsky, C. Tortora
Monthly Notices of the Royal Astronomical Society, Volume 405, Issue 4, July 2010, pp. 2351–2371
We examine correlations between the masses, sizes, and star formation histories for a large sample of
low-redshift early-type galaxies, using a simple suite of dynamical and stellar populations models.
confirm an anti-correlation between size and stellar age, and survey for trends with the central content of dark matter (DM). An average relation between central DM density and galaxy size of
<rhoDM> ~ Reff-2 provides the first clear indication of cuspy
DM haloes in these galaxies – akin to standard LCDM haloes that have undergone adiabatic contraction.
The DM density scales with galaxy mass as expected, deviating from suggestions of a universal halo
profile for dwarf and late-type galaxies. We introduce a new fundamental constraint on galaxy
formation by finding that the central DM fraction decreases with stellar age. This result is only
partially explained by the size-age dependencies, and the residual trend is in the opposite direction
to basic DM halo expectations. Therefore we suggest that there may be a connection between age and
halo contraction, and that galaxies forming earlier had stronger baryonic feedback which expanded
their haloes, or else lumpier baryonic accretion that avoided halo contraction. An alternative
explanation is a lighter initial mass function for older stellar populations.
The globular cluster system of NGC 1399. V. Dynamics of the cluster system out to 80 kpc
Y. Schuberth, T. Richtler, M. Hilker, B. Dirsch, L.P. Bassino, A.J. Romanowsky, L. Infante
Astronomy & Astrophysics, Volume 513, April 2010, Article A52
Globular clusters (GCs) are tracers of the gravitational potential of their host galaxies. Moreover, their kinematic properties may
provide clues for understanding the formation of GC systems and their host galaxies. We use the largest set of GC velocities obtained
so far of any elliptical galaxy to revise and extend the previous investigations (Richtler et al. 2004) of the dynamics of NGC 1399, the
central dominant galaxy of the nearby Fornax cluster of galaxies. The GC velocities are used to study the kinematics, their relation
with population properties, and the dark matter halo of NGC 1399. We have obtained 477 new medium-resolution spectra (of these,
292 are spectra from 265 individual GCs, 241 of which are not in the previous data set). with the VLT FORS 2 and Gemini South
GMOS multi-object spectrographs. We revise velocities for the old spectra and measure velocities for the new spectra, using the
same templates to obtain an homogeneously treated data set. Our entire sample now comprises velocities for almost 700 GCs with
projected galactocentric radii between 6 and 100 kpc. In addition, we use velocities of GCs at larger distances published by Bergond
et al. (2007). Combining the kinematic data with wide-field photometric Washington data, we study the kinematics of the metal-poor
and metal-rich subpopulations. We discuss in detail the velocity dispersions of subsamples and perform spherical Jeans modelling.
The most important results are: The red GCs resemble the stellar field population of NGC 1399 in the region of overlap. The blue
GCs behave kinematically more erratic. Both subpopulations are kinematically distinct and do not show a smooth transition. It is not
possible to find a common dark halo which reproduces simultaneously the properties of both red and blue GCs. Some velocities of blue
GCs are only to be explained by orbits with very large apogalactic distances, thus indicating a contamination with GCs which belong
to the entire Fornax cluster rather than to NGC1399. Also, stripped GCs from nearby elliptical galaxies, particularly NGC1404, may
contaminate the blue sample.
We argue in favour of a scenario in which the majority of the blue cluster population has been accreted during the assembly of the
Fornax cluster. The red cluster population shares the dynamical history of the galaxy itself. Therefore we recommend to use a dark
halo based on the red GCs alone.
The dark halo which fits best is marginally less massive than the halo quoted by Richtler et al. (2004). The comparison with X-ray
analyses is satisfactory in the inner regions, but without showing evidence for a transition from a galaxy to a cluster halo, as suggested
by X-ray work.
Probing the 2D kinematic structure of early-type galaxies out to three effective radii
R.N. Proctor, D.A. Forbes, A.J. Romanowsky, J.P. Brodie, J. Strader, M. Spolaor, J.T. Mendel, L. Spitler
Monthly Notices of the Royal Astronomical Society, Volume 398, Issue 1, September 2009, pp. 91–108
We detail an innovative new technique for measuring the two-dimensional (2D)
velocity moments (rotation velocity, velocity dispersion and Gauss-Hermite coefficients
h3 and h4) of the stellar populations
of galaxy haloes using spectra from Keck DEIMOS (Deep Imaging Multi-Object Spectrograph)
multi-object spectroscopic observations. The data are used to reconstruct 2D rotation velocity maps.
Here we present data for five nearby early-type galaxies to ~three effective radii. We provide
significant insights into the global kinematic structure of these galaxies, and challenge the
accepted morphological classification in several cases. We show that between one and three effective
radii the velocity dispersion declines very slowly, if at all, in all five galaxies. For the two
galaxies with velocity dispersion profiles available from planetary nebulae data we find very
good agreement with our stellar profiles. We find a variety of rotation profiles beyond one
effective radius, i.e rotation speed remaining constant, decreasing and increasing with
radius. These results are of particular importance to studies which attempt to classify galaxies
by their kinematic structure within one effective radius, such as the recent definition of
fast- and slow- rotator classes by the Spectrographic Areal Unit for Research on Optical Nebulae project.
Our data suggest that the rotator class
may change when larger galacto-centric radii are probed. This has important implications for
dynamical modelling of early-type galaxies. The data from this study are available on-line.
Central mass-to-light ratios and dark matter fractions in early-type galaxies
C. Tortora, N.R. Napolitano, A.J. Romanowsky, M. Capaccioli, G. Covone
Monthly Notices of the Royal Astronomical Society, Volume 396, Issue 2, June 2009, pp. 1132–1150
Dynamical studies of local elliptical galaxies and the Fundamental Plane point to a
strong dependence of the total mass-to-light
ratio (M/L) on luminosity with a relation of the form M/L \propto
The 'tilt' γ may be caused by various factors, including stellar population properties
(metallicity, age and star formation history), initial mass function,
rotational support, luminosity profile non-homology and dark matter (DM) fraction. We
evaluate the impact of all these factors using a large uniform dataset of local early-type
Prugniel & Simien. We take particular care in estimating the stellar masses, using a
general star formation history, and comparing different population synthesis models. We find
that the stellar M/L contributes little to the tilt. We estimate the total M/L using simple
Jeans dynamical models, and find that adopting accurate luminosity profiles is important but
does not remove the need for an additional tilt component, which we ascribe to DM. We survey
trends of the DM fraction within one effective radius, finding it to be roughly constant for
galaxies fainter than MB ~ –20.5, and increasing with luminosity for the brighter
galaxies; we detect no significant differences among S0s and fast- and slow-rotating
ellipticals. We construct simplified cosmological mass models and find general consistency,
where the DM transition point is caused by a change in the relation between luminosity and
effective radius. A more refined model with varying galaxy star formation efficiency
suggests a transition from total mass profiles (including DM) of faint galaxies distributed
similarly to the light, to near-isothermal profiles for the bright galaxies. These
conclusions are sensitive to various systematic uncertainties which we investigate in
detail, but are consistent with the results of dynamics studies at larger radii.
Mapping the dark side with DEIMOS: globular clusters, X-ray gas, and dark matter in the NGC 1407 group
A.J. Romanowsky, J. Strader, L.R. Spitler, R. Johnson, J.P. Brodie, D.A. Forbes, T. Ponman
The Astronomical Journal, Volume 137, Number 6, 2009 June, pp. 4956–4987
NGC 1407 is the central elliptical in a nearby evolved group of galaxies
apparently destined to become a galaxy cluster core.
We use the kinematics of globular clusters (GCs) to probe
the dynamics and mass profile of the group's center,
out to a radius of 60 kpc (~10 galaxy effective radii) – the most
extended data set to date around an early-type galaxy.
This sample consists of 172 GC line-of-sight velocities,
most of them newly obtained using Keck/DEIMOS, with
a few additional objects identified as dwarf-globular transition objects or as intragroup GCs.
We find weak rotation for the outer parts of the GC system (v/σ ~ 0.2),
with a rotational misalignment between the metal-poor and metal-rich GCs.
The velocity dispersion profile declines rapidly to a radius of ~20 kpc,
and then becomes flat or rising to ~60 kpc.
There is evidence that the GC orbits have a tangential bias that is
strongest for the metal-poor GCs – in possible contradiction to theoretical expectations.
We construct cosmologically-motivated galaxy+dark halo dynamical models
and infer a total mass within 60 kpc of 3×1012 M☉,
which extrapolates to a virial mass of
~6×1013 M☉ for a typical lambda cold dark matter (ΛCDM) halo – in
agreement with results from kinematics of the group galaxies.
We present an independent Chandra-based analysis,
whose relatively high mass at ~20 kpc disagrees strongly with the GC-based result
unless the GCs are assumed
to have a peculiar orbit distribution, and we therefore discuss more generally some
comparisons between X-ray and optical results.
The group's B-band mass-to-light ratio of
~800 (uncertain by a factor of ~2)
in Solar units is extreme even for a rich galaxy cluster, much less a poor group – placing
it among the most dark matter (DM) dominated systems in the universe,
and also suggesting a massive reservoir of baryons lurking in an unseen phase,
in addition to the nonbaryonic DM.
We compare the kinematical and mass properties of the NGC 1407 group to
other nearby groups and clusters, and
discuss some implications of this system for structure formation.
Dearth of dark matter or massive dark halo? Mass-shape-anisotropy degeneracies revealed by
NMAGIC dynamical models of the elliptical galaxy NGC 3379
F. De Lorenzi, O. Gerhard, L. Coccato, M. Arnaboldi, M. Capaccioli, N.G. Douglas,
K.C. Freeman, K. Kuijken, M.R. Merrifield, N.R. Napolitano, E. Noordermeer,
A.J. Romanowsky, V.P. Debattista
Monthly Notices of the Royal Astronomical Society, Volume 395, Issue 1, May 2009, pp. 76–96
Recent results from the Planetary Nebula Spectrograph (PN.S) survey have revealed
a rapidly falling velocity dispersion profile in the nearby elliptical galaxy NGC
3379, casting doubts on whether this intermediate-luminosity galaxy has the kind
of dark matter halo expected in LCDM cosmology. We present a detailed dynamical
study of this galaxy, combining ground based long-slit spectroscopy, integral-field data
from the SAURON instrument, and PN.S data reaching to more than seven effective
We construct dynamical models with the flexible χ2-made-to-measure particle
method implemented in the NMAGIC code. We fit spherical and axisymmetric models
to the photometric and combined kinematic data, in a sequence of gravitational
potentials whose circular velocity curves at large radii vary between a near-Keplerian
decline and the nearly flat shapes generated by massive halos.
Assuming spherical symmetry we find that the data are consistent both with near-isotropic
systems dominated by the stellar mass, and with models in massive halos
with strongly radially anisotropic outer parts (beta
0.8 at 7Re).
Formal likelihood limits would exclude (at 1 σ)
the model with stars only, as well as halo models with
250 km s–1.
A sequence of more realistic axisymmetric models of different
inclinations and a small number of triaxial tests confirm the spherical results. All
valid models fitting all the data are dynamically stable over Gyrs, including the most
Overall the kinematic data for NGC 3379 out to 7Re
are consistent with a range
of mass distributions in this galaxy. NGC 3379 may well have a dark matter halo
as expected by recent merger models within LCDM cosmology, provided its outer envelope
is strongly radially anisotropic.
Kinematic properties of early-type galaxy haloes using planetary nebulae
L. Coccato, O. Gerhard, M. Arnaboldi, P. Das, N.G. Douglas, K. Kuijken, M.R. Merrifield,
N.R. Napolitano, E. Noordermeer, A.J. Romanowsky, M. Capaccioli M., A. Cortesi,
F. De Lorenzi, K.C. Freeman
Monthly Notices of the Royal Astronomical Society, Volume 394, Issue 3, April 2009, pp. 1249–1283
We present new planetary nebulae (PNe) positions, radial velocities, and magnitudes for 6 early-type
galaxies obtained with the Planetary Nebulae Spectrograph, their two-dimensional velocity and velocity
dispersion fields. We extend this study to include an additional 10 early-type galaxies with PNe
radial velocity measurements available from the literature, to obtain a broader description of the
outer-halo kinematics in early-type galaxies. These data extend the information derived from stellar
kinematics to typically up to ~8 Re.
The combination of photometry, stellar and PNe kinematics shows:
i) good agreement between the PNe number density and the stellar surface brightness in the region
where the two data sets overlap; ii) good agreement between PNe and stellar kinematics; iii) that the
mean rms velocity profiles fall into two groups: with of the galaxies characterized by slowly
decreasing profiles and the remainder having steeply falling profiles; iv) a larger variety of
velocity dispersion profiles; v) that twists and misalignments in the velocity fields are more
frequent at large radii, including some fast rotators; vi) that outer haloes are characterised by more
complex radial profiles of the specific angular momentum-related λR
parameter than observed
within 1 Re; vii) that many objects are more rotationally dominated at large radii
than in their
central parts; and viii) that the halo kinematics are correlated with other galaxy properties, such as
total luminosity, isophotal shape, total stellar mass, V/σ, and alpha parameter,
with a clear
separation between fast and slow rotators.
The Planetary Nebula Spectrograph elliptical galaxy survey: the dark matter in NGC 4494
N.R. Napolitano, A.J. Romanowsky, L. Coccato, M. Capaccioli,
N.G. Douglas, E. Noordermeer, O. Gerhard, M. Arnaboldi, F. De Lorenzi, K. Kuijken,
M.R. Merrifield, E. O'Sullivan, A. Cortesi, P. Das, K.C. Freeman
Monthly Notices of the Royal Astronomical Society, Volume 393, Issue 2, February 2009, pp. 329–353
We present new Planetary Nebula Spectrograph observations of the ordinary elliptical galaxy NGC 4494,
resulting in positions and velocities of 255 PNe out to 7 effective radii (25 kpc).
We also present new wide-field surface photometry from MMT/Megacam, and long-slit stellar kinematics from
VLT/FORS2. The spatial and kinematical distributions of the PNe agree with the field stars in the region
of overlap. The mean rotation is relatively low, with a possible kinematic axis twist outside 1
Re. The velocity dispersion profile declines with radius, though not very steeply,
down to ~70 km/s at the last data point. We have constructed spherical dynamical models of the system,
including Jeans analyses with multi-component LCDM-motivated galaxies as well as logarithmic potentials.
These models include special attention to orbital anisotropy, which we constrain using fourth-order
velocity moments. Given several different sets of modelling methods and assumptions, we find consistent
results for the mass profile within the radial range constrained by the data. Some dark matter (DM) is
required by the data; our best-fit solution has a radially anisotropic stellar halo, a plausible stellar
mass-to-light ratio, and a DM halo with an unexpectedly low central density. We find that this result
does not substantially change with a flattened axisymmetric model. Taken together with other results for
galaxy halo masses, we find suggestions for a puzzling pattern wherein most intermediate-luminosity
galaxies have very low concentration halos, while some high-mass ellipticals have very high
concentrations. We discuss some possible implications of these results for DM and galaxy formation.
Testing the nature of S0 galaxies using planetary nebula kinematics in NGC 1023
E. Noordermeer, M.R. Merrifield, L. Coccato, M. Arnaboldi, M. Capaccioli, N.G. Douglas, K.C. Freeman, O. Gerhard, K. Kuijken, F. De Lorenzi, N.R. Napolitano, A.J. Romanowsky
Monthly Notices of the Royal Astronomical Society, Volume 384, Issue 3, March 2008, pp. 943–952
We investigate the manner in which lenticular galaxies are formed by studying their stellar
kinematics: an S0 formed from a fading spiral galaxy should display similar cold outer disc
kinematics to its progenitor, while an S0 formed in a minor merger should be more dominated by
random motions. In a pilot study to attempt to distinguish between these scenarios, we have
measured the planetary nebula (PN) kinematics of the nearby S0 system NGC 1023. Using the
Planetary Nebula Spectrograph, we have detected and measured the line-of-sight velocities of 204
candidate PNe in the field of this galaxy. Out to intermediate radii, the system displays the
kinematics of a normal rotationally-supported disc system. After correction of its rotational
velocities for asymmetric drift, the galaxy lies just below the spiral galaxy Tully-Fisher
relation, as one would expect for a fading system. However, at larger radii the kinematics
undergo a gradual but major transition to random motion with little rotation. This transition
does not seem to reflect a change in the viewing geometry or the presence of a distinct halo
component, since the number counts of PNe follow the same simple exponential decline as the
stellar continuum with the same projected disc ellipticity out to large radii. The galaxy's small
companion, NGC 1023A, does not seem to be large enough to have caused the observed modification
either. This combination of properties would seem to indicate a complex evolutionary history in
either the transition to form an S0 or in the past life of the spiral galaxy from which the S0
formed. More data sets of this type from both spirals and S0s are needed in order to definitively
determine the relationship between these types of system.
The dark matter halo of NGC 1399 – CDM or MOND?
T. Richtler, Y. Schuberth, M. Hilker, B. Dirsch, L. Bassino, A.J. Romanowsky
Astronomy & Astrophysics, Volume 478, Number 2, February I 2008, pp. L23–L26
Central galaxies in galaxy clusters may be key discriminants in the competition between the cold
dark matter (CDM) paradigm and modified Newtonian dynamics (MOND). We investigate the dark halo
of NGC 1399, the central galaxy of the Fornax cluster, out to a galactocentric distance of 80 kpc.
The data base consists of 656 radial velocities of globular clusters obtained with MXU/VLT and
GMOS/Gemini, which is the largest sample so far for any galaxy. We performed a Jeans analysis for
a non-rotating isotropic model. An NFW halo with the parameters rs = 50 kpc and rhos = 0.0065
M☉/pc3 provides a good description of our data, fitting well to the X-ray mass. More massive
halos are also permitted that agree with the mass of the Fornax cluster as derived from galaxy
velocities. We compare this halo with the expected MOND models under isotropy and find that
additional dark matter on the order of the stellar mass is needed to get agreement. A fully
radial infinite globular cluster system would be needed to change this conclusion. Regarding CDM,
we cannot draw firm conclusions. To really constrain a cluster wide halo, more data covering a
larger radius are necessary. The MOND result appears as a small-scale variant of the finding that
MOND in galaxy clusters still needs dark matter.
The star pile in Abell 545
R. Salinas, T. Richtler, A.J. Romanowsky, M.J. West, Y. Schuberth
Astronomy & Astrophysics, Volume 475, Number 2, November IV 2007, pp. 507–512
Struble (1988, ApJ, 330, L25) found what appeared to be a cD halo without cD galaxy in the center of the galaxy cluster Abell 545. This remarkable feature has been passed almost unnoticed for nearly twenty years.
Our goal is to review Struble's claim by providing a first (preliminary) photometric and spectroscopic analysis of this "star pile".
Based on archival VLT-images and long-slit spectra obtained with Gemini-GMOS, we describe the photometric structure and measure the redshift of the star pile and of the central galaxy.
The star pile is indeed associated with Abell 545. Its velocity is higher by about 1300 km/s than that of the central object. The spectra indicate an old, presumably metal-rich population. Its brightness profile is much shallower than that of typical cD-galaxies.
The formation history and the dynamical status of the star pile remain elusive, until high S/N spectra and a dynamical analysis of the galaxy cluster itself become available. We suggest that the star pile might provide an interesting test of the Cold Dark Matter paradigm.
The PN.S Elliptical Galaxy Survey: data reduction, planetary nebula catalog, and basic dynamics for NGC 3379
N.G. Douglas, N.R. Napolitano, A.J. Romanowsky, L. Coccato, K. Kuijken, M.R. Merrifield, M. Arnaboldi,
O. Gerhard, K.C. Freeman, H.R. Merrett, E. Noordermeer, M. Capaccioli
The Astrophysical Journal, Volume 664, 20 July 2007, pp. 257–276
We present results from Planetary Nebula Spectrograph (PN.S) observations of the elliptical galaxy
NGC 3379 and a description of the data reduction pipeline. We detected 214 planetary nebulae of which
191 are ascribed to NGC 3379, and 23 to the companion galaxy NGC 3384. Comparison with data from the
literature show that the PN.S velocities have an internal error of <20 km/s and a possible offset of
similar magnitude. We present the results of kinematic modeling and show that the PN kinematics are
consistent with absorption-line data in the region where they overlap. The resulting combined
kinematic data set, running from the center of NGC 3379 out to more than seven effective radii
(Reff), reveals a mean rotation velocity that is small compared to the random
velocities, and a dispersion profile that declines rapidly with radius. From a series of Jeans
dynamical models we find the B-band mass-to-light ratio inside 5 Reff to be
8 to 12 in solar units, and the dark matter fraction inside this radius to be less than 40%. We
compare these and other results of dynamical analysis with those of dark-matter-dominated merger
simulations, finding that significant discrepancies remain, reiterating the question of whether NGC
3379 has the kind of dark matter halo that the current ΛCDM paradigm requires.
The Araucaria Project. An accurate distance to NGC 6822 from near-infrared
photometry of Cepheid variables
W. Gieren, G. Pietrzynski, J. Nalewajko, I. Soszynski, F. Bresolin,
R.-P. Kudritzki, D. Minniti, A. Romanowsky
The Astrophysical Journal, Volume 647, 20 August 2006, pp. 1056–1064
We have measured near-infrared magnitudes in the J and K bands for 56
Cepheid variables in the Local Group galaxy NGC 6822 with well-determined periods and
optical light curves in the V and I bands. Using the template light-curve
approach of Soszyński and coworkers, accurate mean magnitudes were obtained from
these data, which allowed us to determine with unprecedented accuracy the distance to
NGC 6822 from a multiwavelength period-luminosity solution in the VIJK bands.
From our data, we obtain a distance to NGC 6822 of (m - M)0 =
23.312 ± 0.021 (random error) mag, with an additional systematic uncertainty of ~3%.
This distance value is tied to an assumed LMC distance modulus of 18.50. From our
multiwavelength approach, we find for the total (average) reddening to the NGC 6822
Cepheids E(B - V) = 0.356 ± 0.013 mag, which is in excellent
agreement with a previous determination by McGonegal and coworkers from near-infrared
photometry and implies significant internal reddening of the Cepheids in NGC 6822. Our
present, definitive distance determination of NGC 6822 from Cepheids agrees within 2%
with the previous distance we had derived from optical photometry alone, but has
significantly reduced error bars. Our Cepheid distance to NGC 6822 is in excellent
agreement with the recent independent determination of Cioni & Habing from the
I-band magnitude of the tip of the red giant branch. It also agrees well, within
the errors, with the early determination of McGonegal et al. (1983) from random-phase
H-band photometry of nine Cepheids.
A deep kinematic survey of planetary nebulae in the Andromeda Galaxy using the Planetary Nebula Spectrograph
H. R. Merrett, M. R. Merrifield, N. G. Douglas, K. Kuijken, A. J. Romanowsky,
N. R. Napolitano, M. Arnaboldi, M. Capaccioli, K. C. Freeman, O. Gerhard, L. Coccato,
D. Carter, N. W. Evans, M. I. Wilkinson, C. Halliday, T. J. Bridges
Monthly Notices of the Royal Astronomical Society, Volume 369, Issue 1, June 2006, pp. 120–142
We present a catalogue of positions, magnitudes and velocities for 3300 emission-line objects found by the Planetary Nebula Spectrograph in a survey of the Andromeda Galaxy, M31. Of these objects, 2615 are found likely to be planetary nebulae (PNe) associated with M31. Initial results from this survey include: the likely non-existence of Andromeda VIII; a universal PN luminosity function, with the exception of a small amount of obscuration, and a small offset in normalization between bulge and disk components; very faint kinematically-selected photometry implying no cut-off in the disk to beyond 4 scalelengths and no halo population in excess of the bulge out to 10 effective bulge radii; disk kinematics that show significant dispersion and asymmetric drift out to large radii, consistent with a warm flaring disk; and no sign of any variation in kinematics with PN luminosity, suggesting that PNe arise from a fairly uniform population of old stars.
Planetary nebula velocities in the disk and bulge of M31
C. Halliday, D. Carter, T. J. Bridges, Z. C. Jackson, M. I. Wilkinson,
D. P. Quinn, N. W. Evans, N. G. Douglas, H. R. Merrett, M. R. Merrifield,
A. J. Romanowsky, K. Kuijken, M. J. Irwin
Monthly Notices of the Royal Astronomical Society, Volume 369, Issue 1, June 2006, pp. 97–119
We present radial velocities for a sample of 723 planetary nebulae (PNe) in the disk and bulge of M31, measured using the WYFFOS fibre spectrograph on the William Herschel telescope. Velocities are determined using the [OIII] 5007 Angstrom emission line. Rotation and velocity dispersion are measured to a radius of 50 arcminutes (11.5 kpc), the first stellar rotation curve and velocity dispersion profile for M31 to such a radius. Our kinematics are consistent with rotational support at radii well beyond the bulge effective radius of 1.4kpc, although our data beyond a radius of 5kpc are limited. We present tentative evidence for kinematic substructure in the bulge of M31 to be studied fully in a later work. This paper is part of an ongoing project to constrain the total mass, mass distribution and velocity anisotropy of the disk, bulge and halo of M31.
Wide-field kinematics of globular clusters in the Leo I group
G. Bergond, S. E. Zepf, A. J. Romanowsky, R. M. Sharples, K. L. Rhode
Astronomy & Astrophysics, Volume 448, Number 1, March II 2006, pp. 155–164
We present wide-field spectroscopy of globular clusters around the
Leo I group galaxies NGC 3379 and NGC 3384 using the FLAMES multi-fibre instrument at the VLT.
We obtain accurate radial velocities for 42 globular clusters (GCs) in
total, 30 for GCs around the elliptical NGC 3379, eight around the
lenticular NGC 3384, and four which may be associated with either galaxy.
These data are notable for their large radial range extending from
0.7' to 14.5' (2 to 42 kpc) from the centre of NGC 3379, and
small velocity uncertainties of about 10 km s–1.
We combine our sample of 30 radial velocities for globular
clusters around NGC 3379 with 8 additional GC
velocities from the literature, and find a projected velocity dispersion of
= 175+24-22 km s–1 at R < 5' and
= 147+44-39 at R > 5'.
These velocity dispersions are consistent with a
dark matter halo around NGC 3379 with a concentration in
the range expected from a ΛCDM cosmological model
and a total mass of ~6×1011 M☉.
Such a model is also a consistent with the stellar velocity dispersion
at small radii and the rotation of the HI ring at large radii,
and has a (M/L)B that increases by
a factor of five from several kpc to 100 kpc.
Our velocity dispersion for the globular cluster system of NGC 3379 is
somewhat higher than that found for the
planetary nebulae (PNe) in the inner region covered by the PN data,
and we discuss possible reasons for this difference.
For NGC 3384, we find the GC system has a rotation signature broadly similar
to that seen in other kinematic probes of this SB0 galaxy. This suggests that
significant rotation may not be unusual in the GC systems of disc galaxies.
Mass-to-light ratio gradients in early-type galaxy haloes
N. R. Napolitano, M. Capaccioli, A. J. Romanowsky, N. G. Douglas,
M. R. Merrifield, K. Kuijken, M. Arnaboldi, O. Gerhard, K. C. Freeman
Monthly Notices of the Royal Astronomical Society, Volume 357, Issue 2, February 2005, pp. 691–706
(also here) |
Owing to the fact that the near future should see a rapidly expanding set of probes of the halo masses of
individual early-type galaxies,
we introduce a convenient parameter for characterising the halo masses from both
observational and theoretical results:
\dML, the logarithmic radial gradient of the mass-to-light ratio.
Using halo density profiles from Λ-cold dark matter (CDM) simulations,
we derive predictions for this gradient for various galaxy luminosities and
star formation efficiencies eSF.
As a pilot study, we
assemble the available \dML\ data from kinematics in early-type galaxies – representing
the first unbiased study of halo masses in a wide range of early-type galaxy luminosities – and
find a correlation between luminosity and \dML, such that the brightest
galaxies appear the most dark-matter dominated.
We find that the gradients in most of the brightest galaxies may fit in well with
the ΛCDM predictions,
but that there is also a population of fainter galaxies whose gradients are so low
as to imply an unreasonably high star formation efficiency eSF > 1.
This difficulty is eased if the dark haloes are not assumed to have the standard
ΛCDM profiles, but instead have lower central concentrations.
Tracing the star stream through M31 using planetary nebula kinematics
H. R. Merrett, K. Kuijken, M. R. Merrifield, A. J. Romanowsky, N. G. Douglas,
N. R. Napolitano, M. Arnaboldi, M. Capaccioli, K. C. Freeman, O. Gerhard,
N. W. Evans, M. I. Wilkinson, C. Halliday, T. J. Bridges, D. Carter
Monthly Notices of the Royal Astronomical Society, Volume 346, Issue 4, December 2003, pp. L62–L66
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We present a possible orbit for the Southern Stream of stars in M31,
which connects it to the Northern Spur. Support for this model comes
from the dynamics of planetary nebulae (PNe) in the disk of M31:
analysis of a new sample of 2611 PNe obtained using the Planetary
Nebula Spectrograph reveals ~20 objects whose kinematics are inconsistent
with the normal components of the galaxy, but which lie at the right
positions and velocities to connect the two photometric features via
this orbit. The satellite galaxy M32 is coincident with the stream both
in position and velocity, adding weight to the hypothesis that the
stream comprises its tidal debris.
A dearth of dark matter in ordinary elliptical galaxies
A. J. Romanowsky, N. G. Douglas, M. Arnaboldi, K. Kuijken, M. R. Merrifield,
N. R. Napolitano, M. Capaccioli, & K. C. Freeman
Science, Volume 301, Issue 5640, 19 September 2003, pp. 1696–1698
free Science Online text |
free Science Online abstract |
Science Express |
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Planetary Nebula Spectrograph homepage
The kinematics of the outer parts of three intermediate-luminosity
elliptical galaxies were studied with the Planetary Nebula
Spectrograph. The galaxies' velocity dispersion profiles were found to
decline with the radius, and dynamical modeling of the data indicates
the presence of little if any dark matter in these galaxies' halos.
This unexpected result conflicts with findings in other galaxy types
and poses a challenge to current galaxy formation theories.
The Planetary Nebula Spectrograph: the green light for galaxy kinematics
N. G. Douglas, M. Arnaboldi, K. C. Freeman, K. Kuijken, M. R. Merrifield, A. J. Romanowsky, K. Taylor, M. Capaccioli, T. Axelrod, R. Gilmozzi, J. Hart, G. Bloxham, & D. Jones
Publications of the Astronomical Society of the Pacific, Volume 114, Number 801, November 2002, pp. 1234–1251
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Planetary nebulae (PNe) are now well established as probes
of galaxy dynamics and as standard candles in distance
determinations. Motivated by the need to improve the efficiency
of planetary nebulae searches and the speed with which their
radial velocities are determined, a dedicated instrument - the
Planetary Nebulae Spectrograph or PN.S
- has been designed and commissioned at the 4.2m
William Herschel Telescope. The high optical efficiency of the
spectrograph results in the detection of typically ~150 PNe
in galaxies at the distance of the Virgo cluster in one night of
observations. In the same observation the radial
velocities are obtained with an accuracy of ~20 km s–1.
Dynamics of stars and globular clusters in M87
Aaron J. Romanowsky & Christopher S. Kochanek
The Astrophysical Journal, Volume 553, Number 2, 1 June 2001, pp. 722–732
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Image of galaxy #1 |
Image of galaxy #2 |
Image of galaxy #3 |
Radio images |
Image of disk |
Virgo cluster in X-rays |
Hubble Heritage page
Surface brightness |
Velocity profiles |
Globular clusters |
We examine the dynamics of the stars and globular clusters in the
nearby giant galaxy M87
and constrain the mass distribution,
using all the available data
over a large range of radii, including higher-order moments of the stellar
line-of-sight velocity distributions
and the discrete velocities of over two hundred globular clusters.
We introduce an extension of spherical orbit modeling methods
that makes full use of all the information in the data,
and provides very robust constraints on the mass models.
We conclusively rule out a constant mass-to-light ratio model,
and infer that the radial density profile of the galaxy's dark halo
falls off more slowly than r-2,
suggesting that the potential of the Virgo Cluster is already dominant at
r ~ 300" ~ 20 kpc.
H0 from the central velocity dispersions of lens galaxies
Aaron J. Romanowsky & Christopher S. Kochanek
The Astrophysical Journal, Volume 516, Number 1, 1 May 1999, pp. 18–26
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Q0957+561 data and images |
PG 1115+080 data, images, info:
We employ Schwarzschild's method of orbit modeling to
constrain the mass profiles of
the central lens galaxies in Q0957+561 and PG 1115+080.
We combine the measured central projected stellar velocity dispersions
of these galaxies with the self-similar radial profiles
of the rms velocity and of the Gauss-Hermite moment h4
observed in nearby galaxies for
0 R 2 Reff.
For Q0957+561, we find a 16% uncertainty in the galaxy mass, and
formal 2-σ limits on the Hubble constant of
H0 = 61-15+13 km s–1 Mpc–1.
For PG 1115+080, we find that none of the viable lens models can be ruled out,
so that H0 is not yet strongly constrained by this system.
Twisting of X-ray isophotes in triaxial galaxies
Aaron J. Romanowsky & Christopher S. Kochanek
The Astrophysical Journal, Volume 493, Number 2, 1 February 1998, pp. 641–649
NGC 720 update
We investigate X-ray isophote twists created by triaxiality differences between
the luminous stellar distributions and the dark halos in elliptical galaxies.
For a typically oblate luminous galaxy embedded in a more prolate halo formed
by dissipationless collapse, the triaxiality difference of
ΔT ≅ 0.7 leads to typical isophote twists of
<Δψ> ≅ 16° ± 19°
at 3 stellar effective radii.
In a model which includes baryonic dissipation
the effect is smaller, with
ΔT ≅ 0.3 and <Δψ> ≅ 5° ± 8°.
Thus, accurate measurements of X-ray isophote twists may be able to
set constraints on the interactions between baryons and dissipationless dark matter
during galaxy formation.
The 30° X-ray isophote twist in the E4 galaxy NGC 720 cannot be reproduced by our model,
suggesting an intrinsic misalignment between the halo and the stars
rather than a projection effect.
Structural and dynamical uncertainties in modelling axisymmetric elliptical galaxies
Aaron J. Romanowsky & Christopher S. Kochanek
Monthly Notices of the Royal Astronomical Society, Volume 287, Issue 1, 1 May 1997, pp. 35–50
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NGC 7619 images
Quantitative dynamical models of galaxies require deprojection of the observed surface
brightness to determine the luminosity density of the galaxy. Existing deprojection
methods for axisymmetric galaxies assume that a unique deprojection exists for any
given inclination, even though the projected density is known to be degenerate to
the addition of 'konus densities' that are invisible in projection. We develop
a deprojection method based on linear regularization that can explore the range of
luminosity densities that are statistically consistent with an observed surface
brightness distribution. The luminosity density is poorly constrained
at modest inclinations (i 30° ),
even in the limit of vanishing observational errors.
In constant mass-to-light ratio, axisymmetric, two-integral
dynamical models, the uncertainties in the luminosity density result in large
uncertainties in the meridional plane velocities.
However, the projected line-of-sight velocities show variations
comparable to current typical observational uncertainties.
Structure and substructure of galactic spheroids
A. J. Romanowsky, J. P. Brodie, J. S. Bullock, R. Ciardullo, P. Guhathakurta, L. Hoffman, K. A. G. Olsen, J. R. Primack, G. van den Ven
Astro2010 Science White Paper, arXiv:0902.3025
The full spatio-chemo-dynamical structure of galaxies of all types and environments at low redshift provides a critical accompaniment to observations of galaxy formation at high redshift. The next decade brings the observational opportunity to strongly constrain nearby galaxies' histories of star formation and assembly, especially in the spheroids that comprise the large majority of the stellar mass in the Universe but have until now been difficult to study. In order to constrain the pathways to building up the spheroidal "red-sequence", various standard techniques in photometry and spectroscopy, particularly with resolved tracer populations like globular clusters and planetary nebulae, can be scaled up to comprehensive surveys as improved wide-field instrumentation is increasingly available. At the same time, progress in adaptive optics on giant telescopes could for the first time permit deep, resolved photometric and spectroscopic analysis of large samples of individual stars in these systems, thereby revolutionizing galaxy studies. Strong theoretical support is needed in order to understand the new observational constraints via detailed modeling and self-consistent simulations of star and galaxy formation throughout cosmic time.
The star formation histories of disk and E/S0 galaxies from resolved stars
K. A. G. Olsen, A. J. Romanowsky, A. Saha, E. Skillman, B. F. Williams, R. F. G. Wyse
Astro2010 Science White Paper, arXiv:0902.4216
The resolved stellar populations of local galaxies, from which it is possible to derive complete star formation and chemical enrichment histories, provide an important way to study galaxy formation and evolution that is complementary to lookback time studies. We propose to use photometry of resolved stars to measure the star formation histories in a statistical sample of galaxy disks and E/S0 galaxies near their effective radii. These measurements would yield strong evidence to support critical questions regarding the formation of galactic disks and spheroids. The main technological limitation is spatial resolution for photometry in heavily crowded fields, for which we need improvement by a factor of ~10 over what is possible today with filled aperture telescopes.
The Planetary Nebula Spectrograph successfully commissioned
M. R. Merrifield, N. G. Douglas, K. Kuijken, and A. J. Romanowsky, for the PN.S Consortium
The ING Newsletter, Number 5, October 2001, pp. 17–18
Extragalactic planetary nebula kinematics with the WHT
Aaron J. Romanowsky, Nigel G. Douglas, Magda Arnaboldi, and Konrad Kuijken
The ING Newsletter, Number 4, March 2001, pp. 23–25
The use of planetary nebulae (PNe) as tracer particles is a promising
approach to studying the kinematics of the outskirts of nearby elliptical
galaxies. We report preliminary results from observations and modelling in
the giant elliptical NGC 4472 (= M49), using the AUTOFIB2/WYFFOS
multifibre spectrograph at the 4.2-m William Herschel Telescope. We also
introduce the Planetary Nebula Spectrograph, a new instrument expressly
designed for measuring the kinematics of PNe.
Digging for formational clues in the halos of early-type galaxies
in Hunting for the Dark: The Hidden Side of Galaxy Formation,
ed. V.P. Debattista & C.C. Popescu, in press, arXiv:1001.3138
Many of the fundamental properties of early-type galaxies (ellipticals and lenticulars) can only be
accessed by venturing beyond their oft-studied centers into their large-radius halo regions. Advances in
observations of kinematical tracers allow early-type halos to be increasingly well probed. This review
focuses on recent findings on angular momentum and dark matter content, and discusses some possible
implications for galaxy structure and formation.
The orbital structure of the massive elliptical galaxy, NGC 5846
P. Das, O. Gerhard, L. Coccato, E. Churazov, W. Forman, A. Finoguenov, H. Böhringer,
M. Arnaboldi, M. Capaccioli, A. Cortesi, F. de Lorenzi, N.G. Douglas, K.C. Freeman, K.C.
K. Kuijken, M.R. Merrifield, N.R. Napolitano, E. Noordermeer, A.J. Romanowsky
Astronomische Nachrichten, 2008, Vol. 329, pp. 940–943
We use density and temperature profiles obtained from XMM-Newton observations to derive a potential of
NGC 5846 out to 11 Re, thus probing the mass distribution deep into the halo. The
inferred circular velocity is significantly higher than the extrapolation of dynamical models implying a
halo, more massive than previously thought. Using an I-band surface-brightness profile and a projected
velocity dispersion profile consisting of long-slit kinematic measurements and planetary nebulae (PNe)
velocity dispersions, we solve the Jeans equations, assuming a non-rotating spherical system. The
solutions suggest a highly radially anisotropic galaxy outside 0.7 Re with
beta ~ 0.75.
Probing the early-type galaxy halos using planetary nebulae as kinematic tracers
L. Coccato, O. Gerhard, M. Arnaboldi, P. Das, N.G. Douglas, K. Kuijken, M.R. Merrifield,
N.R. Napolitano, E. Noordermeer, A.J. Romanowsky, M. Capaccioli, A. Cortesi, F. De
Lorenzi, K.C. Freeman
Astronomische Nachrichten, 2008, Vol. 329, p. 912
We present first results of a study of the halo kinematics for a sample of early type galaxies using
planetary nebulae (PNe) as kinematical tracers. PNe allow to extend up to several effective radii
(Re) the information from absorption line kinematics (confined to within 1 or 2
Re), providing valuable information and constraints for merger simulations and galaxy
formation models. We find that the specific angular momentum per unit mass has a more complex radial
dependence when the halo region is taken into account and that the halo velocity dispersion is related to
the total galaxy luminosity, isophotal shape, and number of PNe per unit of luminosity.
Dark-matter content of early-type galaxies with planetary nebulae
N.R. Napolitano, A.J. Romanowsky, L. Coccato, M. Capaccioli, N.G. Douglas,
E. Noordermeer, M.R. Merrifield, K. Kuijken, M. Arnaboldi, O. Gerhard, K.C. Freeman,
F. De Lorenzi, P. Das
Dark Galaxies & Lost Baryons,
Proceedings of the International Astronomical Union, IAU Symposium, Volume 244,
2008, ed. J.I. Davies & M.J. Disney (Cambridge: Cambridge Univ. Press), pp. 289–294
We examine the dark matter properties of nearby early-type galaxies using planetary nebulae
(PNe) as mass probes. We have designed a specialised instrument, the Planetary Nebula
Spectrograph (PN.S) operating at the William Herschel telescope, with the purpose of
measuring PN velocities with best efficiency. The primary scientific objective of this
custom-built instrument is the study of the PN kinematics in 12 ordinary round galaxies.
Preliminary results showing a dearth of dark matter in ordinary galaxies (Romanowsky et al.
2003) are now confirmed by the first complete PN.S datasets. On the other hand early-type
galaxies with a "regular" dark matter content are starting to be observed among the
brighter PN.S target sample, thus confirming a correlation between the global
dark-to-luminous mass virial ratio
and the galaxy luminosity and mass.
Kinematics of globular cluster systems
A. J. Romanowsky
Globular Clusters - Guides to Galaxies,
2009, ed. T. Richtler & S. Larsen (Berlin: Springer), pp. 433–443
I review the field of globular cluster system (GCS) kinematics,
including a brief primer on observational methods.
The kinematical structures of spiral galaxy GCSs
so far appear to be broadly similar.
The inferred rotation and mass profiles of elliptical galaxy halos exhibit
a diversity of behaviors,
requiring more systematic observational and theoretical studies.
Planetary nebulae as mass tracers in galaxies
A. J. Romanowsky
Planetary Nebulae in Our Galaxy and Beyond,
Proceedings of the International Astronomical Union, Symposium #234,
2006, ed. M.J. Barlow & R.H. Mendez
(Cambridge: Cambridge Univ. Press), pp. 341–348
(invited review talk at Planetary Nebulae in our Galaxy and beyond in April 2006)
Planetary nebula are useful kinematic tracers of the stars in all galaxy types. I review recent observationally-driven developments in the study of galaxy mass profiles. These have yielded surprising results on spiral galaxy disk masses and elliptical galaxy halo masses. A key remaining question is the coupling between PNe and the underlying stellar populations.
Probing the environment with galaxy dynamics
A. J. Romanowsky
Groups of Galaxies in the Nearby Universe,
ESO Astrophysics Symposia, 2007, ed. I. Saviane, V.D. Ivanov & J. Borissova
(Berlin: Springer), pp. 385–389
Groups of Galaxies in the Nearby Universe in Dec 2005)
I present various projects to study the halo dynamics of elliptical
galaxies. This allows one to study the outer mass and orbital
distributions of ellipticals in different environments,
and the inner distributions of groups and clusters themselves.
Dynamics in galaxy halos
A. J. Romanowsky
XI IAU Regional Latin American Meeting of Astronomy, 2006,
ed.. L. Infante & M. Rubio, RevMexAA SC, Volume 26, p. 198
(presented at 11th Annual Latin-American Regional IAU Meeting)
The outskirts of galaxies offer crucial clues about the formation history.
There are clear predictions in the ΛCDM paradigm for the distribution of
mass, angular momentum, and orbit types in galaxy halos. I present work
from several interrelated programs to study the dynamics of nearby
galaxy halos – including a systematic survey of ordinary elliptical
galaxies. The observational probes include X-ray emission and the
kinematics of stars, planetary nebulae, and globular clusters, Initial
results imply dark matter halos in L* galaxies with concentrations
too low for ΛCDM, while in brighter galaxies, there appears to be
too much dark matter for the MOND gravitational theory.
NGC 1399 and MOND
T. Richtler, Y. Schuberth, & A. J. Romanowsky
XI IAU Regional Latin American Meeting of Astronomy, 2006,
ed.. L. Infante & M. Rubio, RevMexAA SC, Volume 26, p. 198
(presented at 11th Annual Latin-American Regional IAU Meeting)
Modified Newtonian Dynamics (MOND), having its succeses in the phenomenology
of rotation curves of disk galaxies, is known to not remove the need for
dark matter in galaxy clusters. Does this also apply to central elliptical
galaxies? Our sample of globular cluster velocities in NGC 1399, the centra
galaxy of the Fornax cluster, now comprises 625 objects out to a galactocentric
distance of 100 kpc, extending the sample of Richtler et al. (2004).
In NGC 1399, the deep MOND regimeis realized only at radial distances larger
than 100 kpc and one has to apply partly physically unmotivated interpolations
between the Newtonian and the MONDian regime. For any of the proposed
interpolation schemes, MOND is not able to explain the circular velocity
indicated by the globular cluster analysis, so dark matter is still needed.
Bekenstein's (2004) interpolation, if applied over the full radial range,
might be considered as being consistent with no dark matter. However,
preliminary analyses suggest that Bekenstein's interpolations fails to remove
the need for dark matter in other central galaxies like NGC 3311 or NGC 6166,
so the case of NGC 1399 probably cannot be generalized. Moreover, it predicts
too high circular velocities for NGC 4636.
Elliptical galaxy halo masses from internal kinematics
A. J. Romanowsky
Mass Profiles and Shapes of Cosmological Structures,
EAS Publications Series, Vol. 20, 2006, ed. G. A. Mamon, F. Combes, C. Deffayet, & B. Fort (Paris: EDP Sciences), pp. 119–126
(invited review talk at
XXIst IAP Colloquium in Jul 2005)
The halo masses of nearby individual elliptical galaxies can be
estimated by using the kinematics of their stars, planetary
nebulae, and globular clusters – ideally in combination. With
currently improving coverage of galaxies of ordinary
luminosities and morphologies, systematic trends may be identified. Bright, boxy ellipticals show strong signatures of dark
matter, while faint, disky ones typically do not. The former
result is problematic for the MOND theory of gravity, and the
latter is a challenge to explain in the ΛCDM paradigm of
Planetary nebulae as dynamical tracers: mass-to-light-ratio gradients
in early-type galaxies
N.R. Napolitano, A.J. Romanowsky, M. Capaccioli, K. Kuijken, M.R. Merrifield, N.G. Douglas, M. Arnaboldi, K.C. Freeman & O. Gerhard
Planetary Nebulae beyond the Milky Way, ESO Astrophysics Symposia,
2006, ed. L. Stanghellini, J.R. Walsh & N.G Douglas (Berlin: Springer), pp. 324–328
(presented at Planetary Nebulae Beyond the Milky Way in May 2004)
Planetary Nebulae (PNe) have enabled mass-to-light ratios (M/L) in early-type
galaxies to be constrained to unprecedented distance from the center,
showing in some cases clear evidence of increasing M/L, in other caes
fairly constant M/L. We combine the information obtained from PN kinematics
with radially extended long-slit spectroscopy data in order to constrain
the M/L trends in a heterogeneous sample of early-type galaxies.
We discuss whether these trends are expected in the ΛCDM framework of
Probing halos with PNe: mass and angular momentum in early-type galaxies
A. J. Romanowsky
Planetary Nebulae beyond the Milky Way, ESO Astrophysics Symposia,
2006, ed. L. Stanghellini, J.R. Walsh & N.G Douglas (Berlin: Springer), pp. 294–298
(presented at Planetary Nebulae Beyond the Milky Way in May 2004)
We present an observational survey program
using planetary nebulae, globular clusters, and
X-ray emission to probe the halos of early-type galaxies.
We review evidence for scanty dark matter halos around ordinary
and discuss the possible implications.
We also present measurements of rotation in the halos.
Mapping the stellar dynamics of M31
M. Merrett, M. Merrifield, K. Kuijken, A. Romanowsky, N. Douglas, N. Napolitano, M. Arnaboldi,
M. Capaccioli, K. Freeman, O. Gerhard, D. Carter, N. W. Evans, M. Wilkinson, C. Halliday, T. Bridges
Planetary Nebulae beyond the Milky Way, ESO Astrophysics Symposia,
2006, ed. L. Stanghellini, J.R. Walsh & N.G Douglas (Berlin: Springer), pp. 281–285
(presented at Planetary Nebulae Beyond the Milky Way in May 2004)
Using the Planetary Nebula Spectrograph, we have observed and measured the velocities for some 2764 PNe in the disk and halo of the Andromeda galaxy. Preliminary analysis using a basic ring model shows a rotation curve in good agreement with that obtained from HI data out to ~20kpc. Some substructure has also been detected within the velocity field, which can be modeled as the continuation of the tidal-remnant known as the Southern Stream, as it passes through Andromeda's disk.
Halo masses of early-type galaxies: theory vs observation
A. J. Romanowsky, N. R. Napolitano, M. Capaccioli,
N. G. Douglas, M. R. Merrifield, K. Kuijken, M. Arnaboldi,
O. Gerhard, & K. C. Freeman
Bulletin of the American Astronomical Society, Volume 36, Number 5, 2004, p. 1397
(presented at the
205th meeting of the American Astronomical Society in Jan 2005)
We introduce a mass-to-light gradient parameter to describe
the amount of dark matter surrounding early-type galaxies.
Assembling kinematic data from the literature on the halo
masses of 20 galaxies, we find a correlation between
luminosity and gradient. The gradients do not fit in well
with predictions from ΛCDM, requiring either baryon excess
or diffuse halos
We also describe various programs using planetary nebulae,
globular clusters, and X-rays to jointly determine galaxy
Some dynamical constraints on the halo of NGC 3379 from wide-field spectroscopy of its globular cluster system
S. E. Zepf, G. Bergond, A. J. Romanowsky, K. L. Rhode, & R. M. Sharples
Bulletin of the American Astronomical Society, Volume 36, Number 5, 2004, p. 1397
(presented at the
205th meeting of the American Astronomical Society in Jan 2005)
We present initial results from a study of the dynamics of the outer halo of the nearby elliptical galaxy NGC 3379 based on wide-field spectroscopy of its globular cluster system.
Our study uses the FLAMES multi-object spectrograph at the VLT to determine the radial velocities of candidate gloublar clusters previously identified in CCD Mosaic images of nearby elliptical galaxies.
Over the large radial range of 0.7 to 15 arcminutes (2 to 45 kpc) covered by our study, the velocity dispersion of the globular cluster system suggests the presence of a dark matter halo around NGC 3379, consistent with the observed rotation of an HI ring at larger distances.
A less significant dark matter halo has been suggested by studies of the planetary nebulae, and we discuss possible reasons for this difference.
Galaxy dynamics with the Planetary Nebula Spectrograph
N. R. Napolitano, A. J. Romanowsky, N. G. Douglas, M. Capaccioli, M. Arnaboldi, K. Kuijken, M. R. Merrifield, K. C. Freeman, O. Gerhard
Memorie della Societa Astronomica Italiana Supplement, Volume 5, 2004, pp. 255–260
The Planetary Nebula Spectrograph is a dedicated instrument for measuring radial velocity of individual Planetary Nebulae (PNe) in galaxies. This new instrument is providing crucial data with which to probe the structure of dark halos in the outskirts of elliptical galaxies in particular, which are traditionally lacking of easy interpretable kinematical tracers at large distance from the center. Preliminary results on a sample of intermediate luminosity galaxies have shown little dark matter within ~5 Reff implying halos either not as massive or not as centrally concentrated as CDM predicts (Romanowsky et al. 2003). We briefly discuss whether this is consistent with a systematic trend of the dark matter content with the luminosity as observed in an extended sample of early-type galaxies.
Halo tracers in nearby galaxies
A. J. Romanowsky, N. G. Douglas, K. Kuijken, M. R. Merrifield, M. Arnaboldi,
H. Merrett, N. R. Napolitano, M. Capaccioli, K. C. Freeman, G. Bergond,
R. M. Sharples, S. E. Zepf, K. L. Rhode
Bulletin of the American Astronomical Society, Volume 35, Number 5, 2003, p. 1312
(presented at the
203rd meeting of the American Astronomical Society in Jan 2004)
We present results from various programs studying the halo kinematics of nearby galaxies.
These include more extensive planetary nebula (PN) data and updated mass
determinations of the "ordinary ellipticals" recently found to show little
trace of dark matter.
Our globular cluster study of NGC 3379 independently confirms the low halo
mass of this galaxy.
PN measurements from a larger galaxy sample show weaker rotation than
expected from galaxy mergers.
We also present 2800 PN velocities around M31, and from these data
infer a dynamical connection between the halo stellar streamer,
the northern spur, and M32.
Elliptical galaxies: darkly cloaked or scantily clad?
A. J. Romanowsky, N. G. Douglas, K. Kuijken, M. R. Merrifield, M. Arnaboldi,
N. R. Napolitano, H. Merrett, M. Capaccioli, K. C. Freeman, O. Gerhard.
Dark Matter in Galaxies, IAU Symposium Vol. 220, 2004,
ed. S. Ryder, D. J. Pisano, M. Walker, & K. Freeman (San Francisco: ASP), pp. 165–170
(presented at Dark Matter in Galaxies in July 2003)
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Planetary nebulae (PNe) may be the most promising tracers in the halos of early-type galaxies.
We have used multi-object spectrographs on the WHT and the VLT,
and the new Planetary Nebula Spectrograph on the WHT,
to obtain hundreds of PN velocities in a small sample of nearby galaxies.
These ellipticals show weak halo rotation, which may be consistent with
ab initio models of galaxy formation,
but not with more detailed major merger simulations.
The galaxies near L* show evidence of a universal declining
velocity dispersion profile, and
dynamical models indicate the presence of little dark matter within 5 Reff – implying
halos either not as massive or not as centrally concentrated as CDM predicts.